TSAN finds data races in generated JavaScript code that use
access the SharedArrayBuffer backing store racily. These are races, but
they are OK in the sense that the JavaScript memory model allows for the
potential bad behavior they could introduce (e.g. potentially tearing
reads). Relaxed atomics could be used here instead, but that could
introduce performance regressions.

This change adds TSAN annotations to the TypedArray reads/writes to
prevent TSAN from warning about them.

Bug: chromium:722871
Change-Id: I0776475f02a352b678ade7d32ed6bd4a6be98c36
Reviewed-on: https://chromium-review.googlesource.com/656509
Commit-Queue: Ben Smith <binji@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47929}