In the previous patch, https://codereview.chromium.org/1923973002/, we added a is_fling flag
to indicate if the active fling is happening on either main or impl thread.

Now we only consider about scrolling happening in impl thread and we will use ack states to
indicate if we want to make the touch events uncancelable.

BUG=595327

Review-Url: https://codereview.chromium.org/2479023003
Cr-Commit-Position: refs/heads/master@{#430640}
23 files changed