Don't route mouse wheel events to transparent children (Legacy_RenderWidgetHostHWND).

Don't route mouse wheel events to transparent children (Legacy_RenderWidgetHostHWND).

Mouse wheel events on Windows would get routed to the window at the current location if it
is in the same process. The message is routed via the SendMessage call. Scrolling via the
wheel on a webpage would end up routing the mouse wheel message received for the main window
to the the legacy window which would then callback into the main window. While the SendMessage
call within the same thread works like a function call, processing the whole callstack twice
causes a lot of jank while scrolling.

Fix is to ignore transparent children in the RerouteMouseWheel function.

BUG=462132
Committed: https://crrev.com/00f5034604fa8073643730299362b2b455e4ada0
Cr-Commit-Position: refs/heads/master@{#341047}

[ananta, 2015-07-29 02:28:33]

ananta@chromium.org changed reviewers:
+ sky@chromium.org

[sky, 2015-07-29 17:27:09]

LGTM

[ananta, 2015-07-30 00:58:20]

The CQ bit was checked by ananta@chromium.org

[commit-bot: I haz the power, 2015-07-30 00:58:55]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1261253003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1261253003/1

[commit-bot: I haz the power, 2015-07-30 02:17:49]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-07-30 02:18:27]

Patchset 1 (id:??) landed as
https://crrev.com/00f5034604fa8073643730299362b2b455e4ada0
Cr-Commit-Position: refs/heads/master@{#341047}