Fixed NOTREACHED() in ProcessWebUIMessage().

The browser_options.html page sends a message to Chrome that only has a handler
on ChromeOS. On non-ChromeOS builds this hits a NOTREACHED() in web_ui_impl.cc:

Check failed: false. Unhandled chrome.send("metricsReportingCheckboxAction");

Added a platform test before sending that message to Chrome.

BUG=None

Review URL: https://codereview.chromium.org/68133013

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234864 0039d316-1c4b-4281-b951-d872f2087c98
DISABLE_ flaky autofill dialog tests on Mac for now.

TBR=isherman@chromium.org
BUG=318526
TEST=less flakes

Review URL: https://codereview.chromium.org/68213013

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234865 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 234864
Events ignoring ack disposition receive synthetic acks.

Previously committed (https://codereview.chromium.org/44983003/),
and then reverted (https://codereview.chromium.org/66283005)

Can't reproduce test failures locally or on the trybots, so we'll try committing again.

Logic to handle events which ignore ack disposition has moved from the
|GestureEventFilter| to the |ImmediateInputRouter|, and |WebInputEventTraits|.

This will allow us to make more events ignore their ack disposition, including
touch events.

Gesture event types which will eventually ignore their ack disposition include 
Tap, ScrollBegin, PinchBegin, PinchEnd, etc.

BUG=302852, 275611
TEST=ImmediateInputRouterTest.EventsIgnoringAckDispositionDontWaitForAcks,
ImmediateInputRouterTest.EventsIgnoringAckDispositionStayInOrder
TBR=jdduke@, joi@

Review URL: https://codereview.chromium.org/71073003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234866 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 234866