Fix input event timing metric in the InputRouter

Ack-ignoring event types do not require ack messages from the renderer.
Consequently, any metrics dependent on such messages should not be affected
by these events.

Review URL: https://codereview.chromium.org/115693007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242196 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242196