[gn] Fix setting v8_target_arch default

This moves setting the v8_target_arch default to the shared
v8.gni, so that it's available in build_overrides for
stand-alone mode.

BUG=chromium:605732, chromium:474921
NOTRY=true

Review-Url: https://codereview.chromium.org/2006213002
Cr-Commit-Position: refs/heads/master@{#36465}

BUILD.gn

@@ -69,17 +69,6 @@ declare_args() {
 v8_random_seed = "314159265"
 v8_toolset_for_shell = "host"
 
-if (v8_target_arch == "") {
-  if (is_msan) {
-    # Running the V8-generated code on an ARM simulator is a powerful hack that
-    # allows the tool to see the memory accesses from JITted code. Without this
-    # flag, JS code causes false positive reports from MSan.
-    v8_target_arch = "arm64"
-  } else {
-    v8_target_arch = target_cpu
-  }
-}
-
 if (v8_use_snapshot && v8_use_external_startup_data) {
   snapshot_target = ":v8_external_snapshot"
 } else if (v8_use_snapshot) {

gni/v8.gni

@@ -2,8 +2,21 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//build/config/sanitizers/sanitizers.gni")
+
 declare_args() {
   # V8 generates code for this architecture. If v8_target_arch differs from
   # target_cpu, a simulator will be run.
   v8_target_arch = ""
 }
+
+if (v8_target_arch == "") {
+  if (is_msan) {
+    # Running the V8-generated code on an ARM simulator is a powerful hack that
+    # allows the tool to see the memory accesses from JITted code. Without this
+    # flag, JS code causes false positive reports from MSan.
+    v8_target_arch = "arm64"
+  } else {
+    v8_target_arch = target_cpu
+  }
+}