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Currently |paint_image_entries_| which keeps track of the content ids
being cached for a PaintImage can grow unbounded. Ensure that we keep
purging it as the cached entries are removed from the
|persistent_cache_|.

R=ericrk@chromium.org

Bug: 867939
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Ic43eee96eec42073a3d14512e62d39946585bba4
Reviewed-on: https://chromium-review.googlesource.com/1152492Reviewed-by: Eric Karl <ericrk@chromium.org>
Commit-Queue: Khushal <khushalsagar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578753}