Flip back switch on Threaded Compositing for web tests.

This change make single threaded the default (again). It does not delete
code that was used for running tests with threaded compositing, to
allow easier evaluation of changes related to threaded compositing in
the future. For more information see go/enable-threaded-compositing.

Change-Id: Ibec42ac68af02236c13a0583ead593ffb456a6d8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4918879
Commit-Queue: Philip Rogers <pdr@chromium.org>
Auto-Submit: Yotam Hacohen <yotha@chromium.org>
Reviewed-by: Yotam Hacohen <yotha@chromium.org>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1208815}