Update LTO flags for Mac.

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 607 matching lines @@
       'video_hole%': 0,
 
       # Automatically select platforms under ozone. Turn this off to
       # build only explicitly selected platforms.
       'ozone_auto_platforms%': 1,
 
       # If this is set clang is used as host compiler, but not as target
       # compiler. Always do this by default.
       'host_clang%': 1,
 
-      # Variables to control Link-Time Optimizations (LTO).
-      # Note: the variables must *not* be enabled at the same time.
-      #       In this case LTO would 'merge' the optimization flags
-      #       at link-time which would lead to all code be optimized with -O2.
-      # Enable LTO on the code compiled with -Os.
-      # See crbug.com/407544
+      # Variables to control Link-Time Optimization (LTO).
+      # On Android, the variable use_lto enables LTO on code compiled with -Os,
+      # and use_lto_o2 enables LTO on code compiled with -O2. On other
+      # platforms, use_lto enables LTO in all translation units, and use_lto_o2
+      # has no effect.
+      #
+      # On Linux and Android, when using LLVM LTO, the script
+      # build/download_gold_plugin.py must be run to download a linker plugin.
+      # On Mac, LLVM needs to be built from scratch using
+      # tools/clang/scripts/update.py and the absolute path to
+      # third_party/llvm-build/Release+Asserts/lib must be added to
+      # $DYLD_LIBRARY_PATH to pick up the right version of the linker plugin.
+      #
+      # On Android, the variables must *not* be enabled at the same time.
+      # In this case LTO would 'merge' the optimization flags at link-time
+      # which would lead to all code be optimized with -O2. See crbug.com/407544
       'use_lto%': 0,
-      # Enable LTO on code compiled with -O2.
       'use_lto_o2%': 0,
 
       # Allowed level of identical code folding in the gold linker.
       'gold_icf_level%': 'safe',
 
       # Libxkbcommon usage.
       'use_xkbcommon%': 0,
 
       # Control Flow Integrity for virtual calls.
       # See http://clang.llvm.org/docs/ControlFlowIntegrity.html
@@ skipping 5157 matching lines @@
        ['CXX.host_wrapper', '<(gomadir)/gomacc'],
       ],
     }],
     ['use_lto==1', {
       'target_defaults': {
         'target_conditions': [
           ['_toolset=="target"', {
             'cflags': [
               '-flto',
             ],
+            'xcode_settings': {
+              'LLVM_LTO': 'YES',
+            },
+          }],
+          # Work-around for http://openradar.appspot.com/20356002
+          ['_toolset=="target" and _type!="static_library"', {
+            'xcode_settings': {
+              'OTHER_LDFLAGS': [
+                '-Wl,-all_load',
+              ],
+            },
           }],
         ],
       },
     }],
     ['use_lto==1 and clang==0', {
       'target_defaults': {
         'target_conditions': [
           ['_toolset=="target"', {
             'cflags': [
               '-ffat-lto-objects',
@@ skipping 246 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',
   },
 }