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If a child surface's activation is throttled then use the system default
deadline as the lower bound for the throttling. This ensures that even if a
child does not have activation dependencies, it will still be throttled
by at least the system default deadline.

This CL also adds some debugging methods and comments on FrameDeadline.

Bug: 672962, 890767
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel
Change-Id: If675908fb2657d5d83b5717339a4060e1eb95f77
Reviewed-on: https://chromium-review.googlesource.com/c/1284050Reviewed-by: Saman Sami <samans@chromium.org>
Commit-Queue: Fady Samuel <fsamuel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600093}