Don't create zero-velocity GestureFlingStarts.

Don't create zero-velocity GestureFlingStarts.

Currently trackpad initiated GestureFlingStarts on CrOS can have zero-
velocity, but this causes crashes in the renderer. This CL converts
these events to be GestureFlingCancels (just in case they are issued
in the middle of ongoing fling).

As it is hard to find the sources of such events after the fact, a
DCHECK has been added to catch new pathways if they issue such events.

BUG=627227
Review-Url: https://codereview.chromium.org/2165043002
Cr-Commit-Position: refs/heads/master@{#406678}
(cherry picked from commit e80690ff28bf6db9392ab9b96a31105e1a6d4205)

Committed: https://chromium.googlesource.com/chromium/src/+/1e514f96624ed05d048b7adb87c4a4406be1ad54

[wjmaclean, 2016-07-25 21:15:27]

Description was changed from

==========
Don't create zero-velocity GestureFlingStarts.

Currently trackpad initiated GestureFlingStarts on CrOS can have zero-
velocity, but this causes crashes in the renderer. This CL converts
these events to be GestureFlingCancels (just in case they are issued
in the middle of ongoing fling).

As it is hard to find the sources of such events after the fact, a
DCHECK has been added to catch new pathways if they issue such events.

BUG=627227

Review-Url: https://codereview.chromium.org/2165043002
Cr-Commit-Position: refs/heads/master@{#406678}
(cherry picked from commit e80690ff28bf6db9392ab9b96a31105e1a6d4205)
==========

to

==========
Don't create zero-velocity GestureFlingStarts.

Currently trackpad initiated GestureFlingStarts on CrOS can have zero-
velocity, but this causes crashes in the renderer. This CL converts
these events to be GestureFlingCancels (just in case they are issued
in the middle of ongoing fling).

As it is hard to find the sources of such events after the fact, a
DCHECK has been added to catch new pathways if they issue such events.

BUG=627227

Review-Url: https://codereview.chromium.org/2165043002
Cr-Commit-Position: refs/heads/master@{#406678}
(cherry picked from commit e80690ff28bf6db9392ab9b96a31105e1a6d4205)

Committed:
https://chromium.googlesource.com/chromium/src/+/1e514f96624ed05d048b7adb87c4...
==========

[wjmaclean, 2016-07-25 21:15:28]

Committed patchset #1 (id:1) manually as
1e514f96624ed05d048b7adb87c4a4406be1ad54.