Enable LTO on Linux for the official builds.

build/common.gypi

 # Copyright (c) 2012 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.
 
 # IMPORTANT:
 # Please don't directly include this file if you are building via gyp_chromium,
 # since gyp_chromium is automatically forcing its inclusion.
 {
   # Variables expected to be overriden on the GYP command line (-D) or by
   # ~/.gyp/include.gypi.
@@ skipping 809 matching lines @@
         ['branding=="Chrome" or chromecast==1', {
           'proprietary_codecs%': 1,
         }, {
           'proprietary_codecs%': 0,
         }],
 
         ['buildtype=="Official"', {
           'enable_prod_wallet_service%': 1,
         }],
 
+        # Enable Link Time Optimization for the official Linux Chrome.
+        # This requires LLVM Gold plugin to be downloaded.
+        # See src/tools/clang/scripts/update.py
+        ['OS=="linux" and target_arch=="x64" and buildtype=="Official" and branding=="Chrome" and chromeos==0', {
+           'use_lto%': 1,
+        }],
+
         # Enable hotwording on Chrome-branded ChromeOS builds.
         ['branding=="Chrome" and chromeos==1', {
           'enable_hotwording%': 1,
         }],
 
         ['OS=="android"', {
           'enable_webrtc%': 1,
         }],
 
         ['OS=="ios"', {
@@ skipping 5454 matching lines @@
     # settings in target dicts.  SYMROOT is a special case, because many other
     # Xcode variables depend on it, including variables such as
     # PROJECT_DERIVED_FILE_DIR.  When a source group corresponding to something
     # like PROJECT_DERIVED_FILE_DIR is added to a project, in order for the
     # files to appear (when present) in the UI as actual files and not red
     # red "missing file" proxies, the correct path to PROJECT_DERIVED_FILE_DIR,
     # and therefore SYMROOT, needs to be set at the project level.
     'SYMROOT': '<(DEPTH)/xcodebuild',
   },
 }