Reland: Enable render throttling for OOPIFs

Originally landed here:
https://chromium-review.googlesource.com/c/chromium/src/+/719601

The change caused a racy test to flake on Mac and was reverted. This
patch restores the original CL and fixes the cause of the flakiness in
the problematic test.

Was earlier relanded with a fix to only one of the two tests that had
race conditions.

[email protected]

Bug: 761478, 784236, 785080
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_site_isolation
Change-Id: I947a70b256b0783b2c687c1213c6e00fe06550a8
Reviewed-on: https://chromium-review.googlesource.com/771773
Reviewed-by: Avi Drissman <[email protected]>
Reviewed-by: Ken Buchanan <[email protected]>
Commit-Queue: Ken Buchanan <[email protected]>
Cr-Commit-Position: refs/heads/master@{#516758}
27 files changed