Add flag to force touch events to be passive during fling as a runtime feature.

We are forcing touchstart and first touchmove to be passive during fling as our part of
intervention. Now we add a flag to turn on this feature at runtime.

BUG=595327

Review-Url: https://codereview.chromium.org/2244503003
Cr-Commit-Position: refs/heads/master@{#412094}
10 files changed