Fix crash on GestureTap in Node::commonAncestor

Fix crash on GestureTap in Node::commonAncestor

When doing a tap, if the synthetic mousedown lands on some node, but
changes the DOM such that there's no node at that point afterwards
(eg. a frame scrollbar), then we'll pass a null node pointer to
commonAncestor and crash.

In addition to adding a test for this case (the mousedown case in
gesture-tap-frame-scrollbar) I added tests for a number of similar
cases (null hit test and removed node) that are already working
correctly but could be subject to similar bugs.

Also corrected a minor typo in a related test
(gesture-tap-click-common-ancestor) - doesn't change behavior, just
better reflects the intention of the test (the ancestor node itself
should not be hittable).

BUG=402338
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=180389

[Rick Byers, 2014-08-15 15:52:34]

Zeeshan, PTAL

[Zeeshan Qureshi, 2014-08-15 18:33:17]

lgtm with a nit.

https://codereview.chromium.org/466143005/diff/1/LayoutTests/fast/events/touc...
File LayoutTests/fast/events/touch/gesture/gesture-tap-frame-scrollbar.html
(right):

https://codereview.chromium.org/466143005/diff/1/LayoutTests/fast/events/touc...
LayoutTests/fast/events/touch/gesture/gesture-tap-frame-scrollbar.html:33: //
Tap at the right edge of the frame, where it's scrollbar would live.
typo: s/it's/its

[Rick Byers, 2014-08-15 20:17:14]

https://codereview.chromium.org/466143005/diff/1/LayoutTests/fast/events/touc...
File LayoutTests/fast/events/touch/gesture/gesture-tap-frame-scrollbar.html
(right):

https://codereview.chromium.org/466143005/diff/1/LayoutTests/fast/events/touc...
LayoutTests/fast/events/touch/gesture/gesture-tap-frame-scrollbar.html:33: //
Tap at the right edge of the frame, where it's scrollbar would live.
On 2014/08/15 18:33:17, Zeeshan Qureshi wrote:
> typo: s/it's/its

Thanks, done.

[Rick Byers, 2014-08-15 20:17:21]

The CQ bit was checked by rbyers@chromium.org

[commit-bot: I haz the power, 2014-08-15 20:17:57]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/rbyers@chromium.org/466143005/20001

[commit-bot: I haz the power, 2014-08-15 21:29:26]

Committed patchset #2 (20001) as 180389