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Allows for creation of HEADLESS Contexts, for browsing sessions without
graphical output.

Implements EnableHeadlessRendering() and DisableHeadlessRendering() on
Frames, which is used for processing DOM animation events in non-graphical
Contexts.

Bug: 1025045
Change-Id: I2cd8e428f4b6eadaafe17631b36b6d42b8d4ea25
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1918036Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#721585}