Add gn option 'is_lto' to enable Link Time Optimization builds.

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 50 matching lines @@
 # Args that are in turn dependent on other args must be in a separate
 # declare_args block. User overrides are only applied at the end of a
 # declare_args block.
 declare_args() {
   # Use libc++ (buildtools/third_party/libc++ and
   # buildtools/third_party/libc++abi) instead of stdlibc++ as standard library.
   # This is intended to be used for instrumented builds.
   use_custom_libcxx =
       (is_asan && is_linux) || is_tsan || is_msan || is_ubsan || use_libfuzzer
 
+  # Enable Link Time Optimization (output programs runs faster,
+  # but linking is up to 5-20x slower.
+  is_lto = is_cfi

[brettw, 2016/03/24 20:27:46]

This shouldn't be in this file because it has nothing to do with sanitizers.

+
   use_sanitizer_coverage = use_libfuzzer
 }
 
 using_sanitizer =
     is_asan || is_lsan || is_tsan || is_msan || is_ubsan || is_ubsan_vptr
 
 assert(!using_sanitizer || is_clang,
        "Sanitizers (is_*san) require setting is_clang = true in 'gn args'")
 
 # 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_vptr),
     "Sanitizers should generally be used in release (set is_debug=false).")