[scheduler] Remove expensive task blocking logic.

Do not try to detect and block expensive tasks as it makes both
predictability and performance worse.

This is expected to reduce high percentiles scrolling latency on
Android by 3%.

Design doc:
https://docs.google.com/document/d/1EvAfCyQVCiZe5plx4CDX-CT3vnS-t7n6u9FOkwBm3MM

R=alexclarke@chromium.org,skyostil@chromium.org
BUG=874836

Change-Id: Ic3aa3f78586fe78b98efa9709dcc7bc61e799f3f
Reviewed-on: https://chromium-review.googlesource.com/c/1411882
Commit-Queue: Alexander Timin <altimin@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Reviewed-by: Alex Clarke <alexclarke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623789}
7 files changed