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Most code that calls the UserGestureIndicator knows which thread it is
executing in. Calling isMainThread involves doing a thread id syscall which
can be quite slow, and there are places in the loading code that do this
in a tight loop (e.g. preloading resources).

This patch adds an optional argument to the indicator's static methods which
optionally allow callers to get the gesture state in a thread-safe way.

BUG=348655

Review-Url: https://codereview.chromium.org/2555013004
Cr-Commit-Position: refs/heads/master@{#437941}