Land chromium-side work to clean up handling of v8_target_cpu in the GN build.

build/config/sanitizers/sanitizers.gni

 # Copyright 2015 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.
 
 declare_args() {
   # Compile for Address Sanitizer to find memory bugs.
   is_asan = false
 
   # Compile for Leak Sanitizer to find leaks.
   is_lsan = false
 
-  # Compile for Memory Sanitizer to find uninitialized reads.
-  is_msan = false
+  # is_msan is declared in //build/config/BUILDCONFIG.gn; see the comments
+  # in that file for why that is.
 
   # Compile for Thread Sanitizer to find threading bugs.
   is_tsan = false
 
   # Compile for Undefined Behaviour Sanitizer to find various types of
   # undefined behaviour (excludes vptr checks).
   is_ubsan = false
 
   # Halt the program if a problem is detected.
   is_ubsan_no_recover = false
@@ skipping 97 matching lines @@
 #
 # If you find a use-case where you want to compile a sanitizer in debug mode
 # and have verified it works, ask brettw and we can consider removing it from
 # this condition. We may also be able to find another way to enable your case
 # without having people accidentally get broken builds by compiling an
 # unsupported or unadvisable configurations.
 #
 # For one-off testing, just comment this assertion out.
 assert(!is_debug || !(is_msan || is_ubsan || is_ubsan_null || is_ubsan_vptr),
        "Sanitizers should generally be used in release (set is_debug=false).")
+
+assert(!is_msan || (is_linux && current_cpu == "x64"),
+       "MSan currently only works on 64-bit Linux and ChromeOS builds.")