Add a flag to disable threaded scrolling

Now that most scroll paths are accelerated, we have minimal coverage of
main-thread scrolling performance. Add a flag that disables threaded
scrolling, allowing local testing of and (potentially) perf bot coverage
for main-thread scroll responsiveness.

This depends on the Blink change
https://codereview.chromium.org/564043003/.

BUG=414323

Review URL: https://codereview.chromium.org/571963003

Cr-Commit-Position: refs/heads/master@{#296725}
8 files changed