Expose new optional  Input.SynthesizeRepeatingScroll parameters
Expose new optional  Input.SynthesizeRepeatingScroll parameters

Patch 2 of 4 to fix the overly long delay between
each scroll in smoothness.scrolling_tough_ad_cases caused by the render
thread being unresponsive.

Patch 1: https://codereview.chromium.org/1299643004/
Patch 3: https://codereview.chromium.org/1291513004/
Patch 4: https://codereview.chromium.org/1285133008/

BUG=510398

Review URL: https://codereview.chromium.org/1296993002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@200723 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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