Since we don't build with ARC enabled and without garbage collection,
__weak had no semantic meaning.  Upstream clang wants to change
__weak to have ARC zeroing weak references semantics, but to not
silently change behavior, they're first making it an error to use __weak
without ARC.  Replace __weak with comments so we can update
compilers.

Also remove a workaround that's no longer necessary.

Similar to https://codereview.chromium.org/1407103008/ on OS X.

BUG=550066,228650
R=stuartmorgan@chromium.org
TBR=thestig

Review URL: https://codereview.chromium.org/1424713006 .

Cr-Commit-Position: refs/heads/master@{#357920}