Use the experimental PBQP register allocator for UBSan builds.

third_party/mesa/mesa.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': {
     'generated_src_dir': 'src/chromium_gensrc',
   },
   'target_defaults': {
     'defines': [
@@ skipping 639 matching lines @@
           'cflags': [
             '-fno-builtin-sin',
           ],
         }],
         ['OS=="win"', {
           'defines': [
             # Because we're building as a static library
             '_GLAPI_NO_EXPORTS',
           ],
         }],
-        ['ubsan==1', {
-          # Due to a bug in LLVM (http://llvm.org/bugs/show_bug.cgi?id=21349),
-          # compilation hangs for some Mesa source files. Disable -O2
-          # temporarily until http://crbug.com/426271 is fixed.
-          'cflags!': [
-            '-O2',
-          ],
-        }],
       ],
     },
     # Building this target will hide the native OpenGL shared library and
     # replace it with a slow software renderer.
     {
       'target_name': 'osmesa',
       'type': 'loadable_module',
       'mac_bundle': 0,
       'dependencies': [
         'mesa_headers',
@@ skipping 55 matching lines @@
                 '<(PRODUCT_DIR)/libosmesa.so',
                 '<(SHARED_LIB_DIR)/libosmesa.so',
               ],
             },
           ],
         },
       ],
     }],
   ],
 }