Update GN files w/ changes needed to make sanitizers work.

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
@@ skipping 82 matching lines @@
 if (use_sanitizer_coverage && sanitizer_coverage_flags == "") {
   sanitizer_coverage_flags = "edge,indirect-calls,8bit-counters"
 }
 
 using_sanitizer = is_asan || is_lsan || is_tsan || is_msan || is_ubsan ||
                   is_ubsan_null || is_ubsan_vptr || is_ubsan_security
 
 assert(!using_sanitizer || is_clang,
        "Sanitizers (is_*san) require setting is_clang = true in 'gn args'")
 
+prebuilt_instrumented_libraries_available =
+    is_msan && (msan_track_origins == 0 || msan_track_origins == 2)
+
 # MSan only links Chrome properly in release builds (brettw -- 9/1/2015). The
 # same is possibly true for the other non-ASan sanitizers. But regardless of
 # whether it links, one would normally never run a sanitizer in debug mode.
 # Running in debug mode probably indicates you forgot to set the "is_debug =
 # false" flag in the build args. ASan seems to run fine in debug mode.
 #
 # 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_lsan || is_tsan || is_ubsan ||
                           is_ubsan_null || is_ubsan_vptr),
        "Sanitizers should generally be used in release (set is_debug=false).")