Reland of Enable LTO on Linux for the official builds.

build/toolchain/toolchain.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.
 
 # Toolchain-related configuration that may be needed outside the context of the
 # toolchain() rules themselves.
 
+import("//build/config/chrome_build.gni")
+
 declare_args() {
   # Enable Link Time Optimization in optimized builds (output programs run
   # faster, but linking is up to 5-20x slower).
-  #
-  # TODO(pcc): Remove this flag if/when LTO is enabled in official builds.
-  allow_posix_link_time_opt = false
+  # Note: use target_os == "linux" rather than is_linux so that it does not
+  # apply to host_toolchain when target_os="android".
+  allow_posix_link_time_opt =
+      target_os == "linux" && !is_chromeos && target_cpu == "x64" &&
+      is_chrome_branded && is_official_build

[Dirk Pranke, 2016/05/26 18:26:29]

nit: is_chromeos will never be true of target_os == "linux", so you don't need
that clause.

 
   # Set to true to use lld, the LLVM linker. This flag may be used on Windows
   # with the shipped LLVM toolchain, or on Linux with a self-built top-of-tree
   # LLVM toolchain (see llvm_force_head_revision in
   # build/config/compiler/BUILD.gn).
   use_lld = false
 
   if (is_clang) {
     # Clang compiler version. Clang files are placed at version-dependent paths.
     clang_version = "3.9.0"
@@ skipping 43 matching lines @@
 stamp_description = "STAMP {{output}}"
 copy_description = "COPY {{source}} {{output}}"
 if (host_os == "win") {
   stamp_command = "$python_path gyp-win-tool stamp {{output}}"
   copy_command =
       "$python_path gyp-win-tool recursive-mirror {{source}} {{output}}"
 } else {
   stamp_command = "touch {{output}}"
   copy_command = "ln -f {{source}} {{output}} 2>/dev/null || (rm -rf {{output}} && cp -af {{source}} {{output}})"
 }