Support transitioning between platform scrolling and JS pinching

Support transitioning between platform scrolling and JS pinching

While a scroll gesture sequence is active, dispatch secondary touchstart
events synchronously, allowing them to be preventDefault'ed by the page.
This affords sites the opportunity to customize pinch/multitouch without
completely sacrificing the benefits of platform (and threaded) scrolling.

Note that even after a page preventDefaults a secondary touchstart,
platform scrolling can resume if the handler stops preventDefault'ing
touchmove events.

BUG=447738
Committed: https://crrev.com/6f12081771bb3f89e3d2d6b6e5e91a16c29ab4f6
Cr-Commit-Position: refs/heads/master@{#311737}

[jdduke (slow), 2015-01-15 19:25:56]

jdduke@chromium.org changed reviewers:
+ rbyers@chromium.org

[jdduke (slow), 2015-01-15 19:25:57]

rbyers@: Could you take a look? Tested on http://jsbin.com/xoqapi/1/quiet (I
know, nothing fancy), and you can nicely transition between scrolling and
preventDefault'ing multitouch.

[Rick Byers, 2015-01-15 20:50:31]

Nice, lgtm!

[jdduke (slow), 2015-01-15 20:51:52]

The CQ bit was checked by jdduke@chromium.org

[commit-bot: I haz the power, 2015-01-15 20:52:49]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/844983004/1

[commit-bot: I haz the power, 2015-01-15 21:17:03]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-01-15 21:19:33]

Patchset 1 (id:??) landed as
https://crrev.com/6f12081771bb3f89e3d2d6b6e5e91a16c29ab4f6
Cr-Commit-Position: refs/heads/master@{#311737}