Skip updating nodeUnderMouse to clickTarget.

Skip updating nodeUnderMouse to clickTarget.

After a drag (mousedown-mousemove-mouseup) across an element border,
chromium sends "click" event to the common ancestor of of mousedown &
mouseup nodes. At that time, EventHandler::m_nodeUnderMouse was
mistakenly updated to the common ancestor, resulting in redundant
mouse node-transition events (mouseout/over/enter/leave) during & after
the click. This CL skips updating nodeUnderMouse to fix the bug.

BUG=356090
Committed: https://crrev.com/b729757fd10f845697c3028d5e41dbe4ce1295a0
Cr-Commit-Position: refs/heads/master@{#354519}

[mustaq, 2015-10-14 15:45:28]

The CQ bit was checked by mustaq@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-10-14 15:45:43]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1402133003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1402133003/1

[commit-bot: I haz the power, 2015-10-14 17:22:48]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-10-14 17:22:48]

Dry run: This issue passed the CQ dry run.

[mustaq, 2015-10-14 17:55:39]

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

[mustaq, 2015-10-14 17:55:40]

ptal

[Rick Byers, 2015-10-15 21:24:43]

Nice catch, LGTM

[mustaq, 2015-10-16 14:23:16]

The CQ bit was checked by mustaq@chromium.org

[commit-bot: I haz the power, 2015-10-16 14:23:32]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1402133003/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1402133003/20001

[commit-bot: I haz the power, 2015-10-16 15:53:55]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-10-16 15:54:33]

Patchset 2 (id:??) landed as
https://crrev.com/b729757fd10f845697c3028d5e41dbe4ce1295a0
Cr-Commit-Position: refs/heads/master@{#354519}