Remove mouse-related hit tests during a GestureTap

This further reduces the total number of hit tests in trivial tap scenarios
from 9 to 6.  It looks like all the work we do with un-hit-tested mouse
events is really mouse specific and probably not something we want in the
tap case.  So we can bump our emulation down to a lower level without
having to otherwise refactor the existing code.

This change is observable by web applications.  For example, if a
mousedown event handler modifies the DOM, then previously the mouseup and
click events would be sent to the new element under the point.  Now
all the events that happen as a result of a tap will be sent to the same
node.  Since they all happen at the same conceptual moment, this change
is probably a net improvement.  But we'll need to watch for any
scenarios that regress.

BUG=381728

Review URL: https://codereview.chromium.org/353893002

git-svn-id: svn://svn.chromium.org/blink/trunk@178098 bbb929c8-8fbe-4397-9dbb-9b2b20218538