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Currently, the PaintWorkletImageCache keeps a hashmap from
PaintWorkletInput to a PaintRecord. The problem is that the PaintRecord
could be used by multiple tiles for rasterization, while the main
thread might be trying to purge the cache.

This CL makes the PaintRecord ref counted. Whenever a tile is using it
for rasterization, the counter will increment. When the rasterization
is done, the counter will decrement.

Bug: 907897
Change-Id: Idaf1a1f191c084b7531c3a17e5163288e8ba1d4b
Reviewed-on: https://chromium-review.googlesource.com/c/1439761Reviewed-by: vmpstr <vmpstr@chromium.org>
Commit-Queue: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#626872}