Use the experimental PBQP register allocator for UBSan builds.

build/common.gypi

Index: build/common.gypi
diff --git a/build/common.gypi b/build/common.gypi
index f97bf665566415d4b00747a505ce9d4976c11e67..c11a1dc3cd9d4fd0b0f866a1acaee14a7a502f43 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -4214,6 +4214,13 @@
                   '-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',