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- Force compositing for backdrop-filters. We probably won't support
  non-composited backdrop-filters because it depends on viz renderers.

- Paint empty display item for backdrop-filter to ensure a paint
  chunk which will create the composited backdrop-filter layer.

Bug: 923429
Change-Id: Ib2afbde2c4ad3a485049b25ee63d6537f675ade6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1824317
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: Mason Freed <masonfreed@chromium.org>
Cr-Commit-Position: refs/heads/master@{#700376}