Actually increment the platform timer heap order ID across threads.
Actually increment the platform timer heap order ID across threads.

Timers get a monotonically increasing ID that's used to maintain the
order of timers scheduled with the same time. However the ID
assignment predates Worker threads and now racily update the ID
counter across threads.

This change increments the ID counter in a thread-safe way.

BUG=247310
TEST=Do a TSAN build of Chrome [1] and run the ClusterFuzz repro.
     Verify that TimerBase does not appear in the log.

[1] http://www.chromium.org/developers/testing/threadsanitizer-tsan-v2

Review URL: https://codereview.chromium.org/765013002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@186157 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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