Make v8_target_arch a GN declare_arg().

BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/android/config.gni")
 import("//build/config/arm.gni")
 import("//build/config/mips.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 
 if (is_android) {
@@ skipping 52 matching lines @@
   # Similar to vfp but on MIPS.
   v8_can_use_fpu_instructions = true
 
   # Similar to the ARM hard float ABI but on MIPS.
   v8_use_mips_abi_hardfloat = true
 }
 
 v8_random_seed = "314159265"
 v8_toolset_for_shell = "host"
 
-if (is_msan) {
+declare_args() {
   # 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
+  # allows sanitizers and other tools to see the memory accesses from
+  # JITted code.
+  v8_target_arch = ""

[Michael Achenbach, 2016/05/24 06:39:32]

I've recently added this to v8's overrides:
https://code.google.com/p/chromium/codesearch#chromium/src/v8/build_overrides...

It should be removed there now.

I'm not so sure about the documentation. This seems sanitizer specific and
should go to the is_msan condition below. As main docu maybe:
"V8 generates code for this architecture. If v8_target_arch differs from
target_arch, a simulator will be run."

+}
+
+if (v8_target_arch == "") {
+  if (is_msan) {
+    # 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) {
   snapshot_target = ":v8_snapshot"
 } else {
   assert(!v8_use_external_startup_data)
   snapshot_target = ":v8_nosnapshot"
 }
@@ skipping 2100 matching lines @@
   sources = [
     "test/fuzzer/wasm-asmjs.cc",
   ]
 
   deps = [
     ":fuzzer_support",
   ]
 
   configs = [ ":internal_config" ]
 }