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}