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The event queue is never cleared, so if any events are requested,
they'll leak memory.  This CL adds a check to ensure no events were
ever pending.  It's added as a CHECK(), not a DCHECK(), since we
want to receive crash reports if users with untested GPU drivers
ever hit this.

Change-Id: I2aa2fd9f88a1637deff5e81996de71c1d5956e0f
Bug: 1158170
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2602704
Auto-Submit: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840705}