Use the experimental PBQP register allocator for UBSan builds.

ui/gl/gl.gyp

 # 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.
 
 {
   'variables': {
     'chromium_code': 1,
   },
 
   'targets': [
@@ skipping 294 matching lines @@
           'dependencies': [
             '../ozone/ozone.gyp:ozone',
             '../ozone/ozone.gyp:ozone_base',
           ],
         }],
         ['OS=="android" and android_webview_build==0', {
           'dependencies': [
             '../android/ui_android.gyp:ui_java',
           ],
         }],
-        ['ubsan==1', {
-          # Due to a bug in LLVM (http://llvm.org/bugs/show_bug.cgi?id=21349),
-          # compilation hangs for some GL source files. Disable -O2 temporarily
-          # until http://crbug.com/426271 is fixed.
-          'cflags!': [
-            '-O2',
-          ],
-        }],
       ],
     },
     {
       'target_name': 'gl_unittest_utils',
       'type': 'static_library',
       'variables': {
         'gl_binding_output_dir': '<(SHARED_INTERMEDIATE_DIR)/ui/gl',
       },
       'dependencies': [
         '../../testing/gmock.gyp:gmock',
@@ skipping 42 matching lines @@
           ],
           'variables': {
             'jni_gen_package': 'ui/gl',
           },
           'includes': [ '../../build/jni_generator.gypi' ],
         },
       ],
     }],
   ],
 }