Clang roll 209387:214024 (take 2)

Clang roll 209387:214024 (take 2)

- The previous attempt failed due to crashing layout tests (fast/text-autosize).
  That was fixed in Blink r179171.

- This roll seems to cause a slight binary size increase, perhaps related
  to changed inlining behaviour. On Linux this is offset by the switch to
  Clang just having saved megs of binary size. On Mac this is unfortunate,
  but since we haven't rolled in a long time, we decided not to consider
  it a blocker.

- Note: this version adds optimizations based on exploiting that references
  must refer to valid objects, and so if 'foo' is a reference, checks such
  as "if (&foo == NULL)" can be optimized away.

BUG=381489
R=thakis@chromium.org

Review URL: https://codereview.chromium.org/430603002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@286532 0039d316-1c4b-4281-b951-d872f2087c98