Use the experimental PBQP register allocator for UBSan 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 4196 matching lines @@
               }],
             ],
           }],
           ['ubsan==1', {
             'target_conditions': [
               ['_toolset=="target"', {
                 'cflags': [
                   '-fsanitize=undefined',
                   # -fsanitize=vptr is incompatible with -fno-rtti.
                   '-fno-sanitize=vptr',
+                  # Employ the experimental PBQP register allocator to avoid
+                  # slow compilation on files with too many basic blocks.
+                  # See http://crbug.com/426271.
+                  '-mllvm -regalloc=pbqp',
+                  # Speculatively use coalescing to slightly improve the code
+                  # generated by PBQP regallocator. May increase compile time.
+                  '-mllvm -pbqp-coalescing',
                 ],
                 'ldflags': [
                   '-fsanitize=undefined',
                   # -fsanitize=vptr is incompatible with -fno-rtti.
                   '-fno-sanitize=vptr',
                 ],
               }],
             ],
           }],
           ['ubsan_vptr==1', {
@@ skipping 1697 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',
   },
 }