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The GpuWatchdogThread currently allows multiple tasks to disarm the
watchdog to be posted in a row, only stopping posting when the watchdog
is actually disarmed. This is fairly wasteful, as all but the first
task will just early out. This change adjusts the logic so that we only
post one disarm task, at a time, blocking future disarm tasks until
we have successfully disarmed / rearmed the watchdog.

This change also removes the current "volatile bool", and replaces this with a
base::subtle::Atomic32, which makes the expectations regarding synchronization
more explicit.

BUG=612219
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2389633003
Cr-Commit-Position: refs/heads/master@{#422549}