Enable whole-program virtual function optimization in LTO mode.

tools/mb/mb_config.pyl

 # 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.
 
 {
   # This is the list of configs that you can pass to mb; each config
   # represents a particular combination of GYP_DEFINES/gn args that
   # we must support. A given config *may* be platform-specific but
   # is not necessarily so (i.e., we might have mac, win, and linux
   # bots all using the 'gn_release_bot' config).
@@ skipping 322 matching lines @@
     },
 
     'libfuzzer': { 'gn_args': 'use_libfuzzer=true' },
 
     'lsan': {
       'gn_args': 'is_lsan=true',
       'gyp_defines': 'lsan=1',
     },
 
     'lto': {
-      'gn_args': 'is_lto=true',
+      'gn_args': 'allow_posix_link_time_opt=true',
       'gyp_defines': 'use_lto=1',
     },
 
     'mac_strip': {
       'gn_args': 'error', # TODO(GYP): Port mac_strip_release
       'gyp_defines': 'mac_strip_release=1',
     },
 
     'minimal_symbols': {
       'gn_args': 'symbol_level=1',
@@ skipping 465 matching lines @@
       'win_chromium_clang_upload': 'gn_release_bot',
       'win_clang_x64_dbg': 'win_clang_debug_bot',
     },
     'tryserver.v8': {
       'v8_linux_chromium_gn_rel': 'gn_release_trybot',
       'v8_android_chromium_gn_dbg': 'android_gn_debug_bot',
       'v8_linux_blink_rel': 'swarming_gn_release_trybot_minimal_symbols_x64',
     },
   },
 }