Add a setting to disable threaded scrolling

Add a web setting for disabling threaded scrolling. The
ScrollingCoordinator enforces this by tagging all layers as main-thread
scroll targets.

Such a setting is useful for local testing and potentially for perf
instrumentation to track and improve main-thread driven scrolling. Note
also that this is but a temporary solution as other synchronous
scrolling triggers are explored.

The Chromium flag to trigger this path is added in
https://codereview.chromium.org/571963003/.

BUG=414323,410974

Review URL: https://codereview.chromium.org/564043003

git-svn-id: svn://svn.chromium.org/blink/trunk@182611 bbb929c8-8fbe-4397-9dbb-9b2b20218538