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Currently, Skia flushes unused items out of its caches after ~1 second
of non-use (50 flushes). This works fine while Skia is in-use, but when
a worker context goes idle we stop calling into Skia altogether. This
means that Skia will never flush out unused cache items from the last
piece of work it did. This can lead to some rather large temporaries
being kept around (~20mb).

This change adds an idle cleanup process which flushes Skia's caches
and cleans up worker context resources after a worker context is idle
for 1 second.

BUG=624630
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_precise_blink_rel

Review-Url: https://codereview.chromium.org/2353033003
Cr-Commit-Position: refs/heads/master@{#420496}