Removed a redundant mouse-state from EventHandler.

Removed a redundant mouse-state from EventHandler.

EventHandler currently have two state vars for tracking
node under mouse: m_nodeUnderMouse and m_lastNodeUnderMouse.
These two pointers are used primarily for mouse entry/exit
events, but they point to the same node until we are about
to send mouseenter/leave/over/out events. In the few cases
these pointers become unequal otherwise (happens only when
dispathMouseEvent is called with setUnder=false), it is not
clear why we didn't update the old state. My experiments
suggest that updating the old state doesn't hurt in these
cases. So it makes sense to nuke the m_lastNodeUnderMouse
state.

BUG=

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=201023

[mustaq, 2015-08-19 16:55:19]

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

[commit-bot: I haz the power, 2015-08-19 16:55:34]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1293973004/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1293973004/1

[commit-bot: I haz the power, 2015-08-19 18:05:39]

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

[commit-bot: I haz the power, 2015-08-19 18:05:40]

Dry run: Try jobs failed on following builders:
  win_chromium_rel_ng on tryserver.chromium.win (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.win/builders/win_chromium_rel_...)

[mustaq, 2015-08-19 22:24:58]

Patchset #2 (id:20001) has been deleted

[mustaq, 2015-08-20 14:52:12]

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

[mustaq, 2015-08-20 14:54:50]

ptal

[Rick Byers, 2015-08-21 14:08:32]

Nice simplification!!!

There's of course always some risk around this code that subtle behavior changes
can somehow affect website compat (I've been burned several times myself, and
don't have a ton of confidence in our test coverage).  But attempting
simplifications and learning from them is the only way we'll make progress. 
Thank you!

LGTM

[mustaq, 2015-08-21 20:06:53]

On 2015/08/21 14:08:32, Rick Byers wrote:
> Nice simplification!!!
> 
> There's of course always some risk around this code that subtle behavior
changes
> can somehow affect website compat (I've been burned several times myself, and
> don't have a ton of confidence in our test coverage).  But attempting
> simplifications and learning from them is the only way we'll make progress. 
> Thank you!
> 
> LGTM

Yes, there is some risk. If things break, hopefully we will find a better way to
fix the problem. Thanks.

[mustaq, 2015-08-21 20:07:16]

The CQ bit was checked by mustaq@chromium.org

[commit-bot: I haz the power, 2015-08-21 20:07:43]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1293973004/40001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1293973004/40001

[commit-bot: I haz the power, 2015-08-21 22:44:58]

Committed patchset #2 (id:40001) as
https://src.chromium.org/viewvc/blink?view=rev&revision=201023