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This passes all current css/filters layout tests and skia renderer
backdrop filter pixel test.

Remove the TODO(skaslev): The software renderer does not support filters yet.

BUG=822857,828728

Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel
Change-Id: I6f96259249612e8655c953bdd6a534ae9f5c6acc
Reviewed-on: https://chromium-review.googlesource.com/985635
Commit-Queue: Xing Xu <xing.xu@intel.com>
Reviewed-by: enne <enne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#552260}