MacViews: Fix crash when EventMonitorMac receives a NSSystemDefinedEvent

MacViews: Fix crash when EventMonitorMac receives a system defined event.

EventFromNative() returns null when given a system defined event, so filter them
out when monitoring the event stream.
When running the experimental MacViews browser, I was seeing an event
with type=SysDefined subtype=7 (apparently it means that a mouse button
state has changed).

BUG=425229
Committed: https://crrev.com/40b0d46d7c7660cb8cb8bfc68df1b233f0811cd3
Cr-Commit-Position: refs/heads/master@{#319142}

[Andre, 2015-03-04 21:51:40]

andresantoso@chromium.org changed reviewers:
+ avi@chromium.org

[Andre, 2015-03-04 21:51:41]

Avi, please review.

[Avi (use Gerrit), 2015-03-04 22:00:03]

lgtm

[Andre, 2015-03-04 22:07:42]

The CQ bit was checked by andresantoso@chromium.org

[commit-bot: I haz the power, 2015-03-04 22:08:35]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/981693002/1

[commit-bot: I haz the power, 2015-03-04 22:29:04]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-04 22:30:00]

Patchset 1 (id:??) landed as
https://crrev.com/40b0d46d7c7660cb8cb8bfc68df1b233f0811cd3
Cr-Commit-Position: refs/heads/master@{#319142}