Revert of Re-land "Fix double-building of v8 in GN builds" (patchset #2...

Revert of Re-land "Fix double-building of v8 in GN builds" (patchset #2 id:20001 of https://codereview.chromium.org/2171083003/ )

Reason for revert:
Reverted, because it breaks some funky android build.

Can reproduce breakage locally, w/ args.gn as follows:
disable_brotli_filter = true
disable_file_support = true
disable_ftp_support = true
enable_websockets = false
ffmpeg_branding = "Chrome"
is_component_build = false
is_debug = true
proprietary_codecs = true
symbol_level = 1
target_cpu = "x86"
target_os = "android"
use_goma = true
use_platform_icu_alternatives = true

This ends up building the mkpeephole tool w/ an architecture that won't run on the build machine.

Original issue's description:
> Re-land "Fix double-building of v8 in GN builds"
>
> This re-lands r37926 w/ the needed fix for cross-compiles; we
> can only re-use the default toolchain when the host can actually
> run it.
>
> R=machenbach@chromium.org
> BUG=629825
>
> Committed: https://crrev.com/5b762044b53f988fa9a534fe1a84f9938b3abd75
> Cr-Commit-Position: refs/heads/master@{#37970}

TBR=machenbach@chromium.org,dpranke@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=629825

Review-Url: https://codereview.chromium.org/2175693003
Cr-Commit-Position: refs/heads/master@{#37984}

snapshot_toolchain.gni

@@ -38,46 +38,26 @@ declare_args() {
 # configurations v8 runs on.
 if (v8_snapshot_toolchain == "") {
   if (host_cpu == "x64" && host_os == "linux") {
-    if (v8_current_cpu == v8_target_cpu && current_cpu == target_cpu &&
-        (target_cpu == "x86" || target_cpu == "x64")) {
-      # Args that are specified by the user can end up having the same
-      # effect as args that are specified in a toolchain; for example,
-      # building on linux x64 with target_cpu == "x86" v8_target_cpu == "arm"
-      # will use the //build/toolchain/linux:clang_x86 toolchain, but have the
-      # same effect as if you specified
-      #
-      #   custom_toolchain="//build/toolchain/linux:clang_x86_v8_arm"
-      #
-      # As a result, if we didn't take that into account we could
-      # accidentally end up building two identical copies of v8.
-      #
-      # So, we can re-use the default toolchain for the snapshot in these
-      # casees, but only when the host is actually capable of running the
-      # target code, which at this time means that the target_cpu == x86
-      # or x64.
-      v8_snapshot_toolchain = default_toolchain
+    if (current_cpu == "arm" || current_cpu == "mipsel" ||
+        current_cpu == "x86") {
+      _snapshot_cpu = "x86"
     } else {
-      if (current_cpu == "arm" || current_cpu == "mipsel" ||
-          current_cpu == "x86") {
-        _snapshot_cpu = "x86"
-      } else {
-        assert(current_cpu == "arm64" || current_cpu == "x64" ||
-                   current_cpu == "mips64el",
-               "Need environment for this arch: $current_cpu")
-        _snapshot_cpu = "x64"
-      }
+      assert(current_cpu == "arm64" || current_cpu == "x64" ||
+                 current_cpu == "mips64el",
+             "Need environment for this arch: $current_cpu")
+      _snapshot_cpu = "x64"
+    }
 
-      if (v8_current_cpu != _snapshot_cpu) {
-        _cpus = "${_snapshot_cpu}_v8_${v8_current_cpu}"
-      } else {
-        _cpus = _snapshot_cpu
-      }
+    if (v8_current_cpu != _snapshot_cpu) {
+      _cpus = "${_snapshot_cpu}_v8_${v8_current_cpu}"
+    } else {
+      _cpus = _snapshot_cpu
+    }
 
-      if ((host_os == "linux" && current_os == "android") || is_clang) {
-        v8_snapshot_toolchain = "//build/toolchain/linux:clang_${_cpus}"
-      } else {
-        v8_snapshot_toolchain = "//build/toolchain/linux:${_cpus}"
-      }
+    if ((host_os == "linux" && current_os == "android") || is_clang) {
+      v8_snapshot_toolchain = "//build/toolchain/linux:clang_${_cpus}"
+    } else {
+      v8_snapshot_toolchain = "//build/toolchain/linux:${_cpus}"
     }
   } else if (host_os == "mac" && current_os == "win") {
     assert(v8_current_cpu == current_cpu,