Tentative fix for crash during GestureTap handling

In r178098 I changed EventHandler::HandleGestureTap to re-use only
the needed portions of mouse event handling, rather than fully simulating
a real mouse event.  Since that change we've seen a few crashes reported
inside of HandleGestureTap that look like they could be due to a null
m_frame->document() or null m_frame->page().  I've tried unsuccessfully
to reproduce such crashes, but I noticed that the original mouse handling
code adds a ref to the FrameView for the duration of the function. I've
re-added that ref in HandleGestureTap in the hopes it'll address these
crashes.

I've also added a test that verifies removing an iframe during the
middle of a tap being handled in it doesn't cause any problems. I wasn't
able to get this to crash, but it's still good test coverage to have.

BUG=399020,398982

Review URL: https://codereview.chromium.org/433953002

git-svn-id: svn://svn.chromium.org/blink/trunk@179337 bbb929c8-8fbe-4397-9dbb-9b2b20218538