Minimal fix to gesture event frame targetting

We intended to handle gesture events in the frame of the best touch
adjustment node.  But HitTestResult::resolveRectBaseTest failed to
update the m_innerNonSharedNode field, leaving just the last candidate
from the rect-based hit test.  EventHandler::handleGestureEvent then
ended up using this via HitTestResult::innerNodeFrame() sending events
to possibly the incorrect frame.

This is a low-risk fix for merging back to M38 which just sets the
missing m_innerNonSharedNode field.  But there are deeper pre-existing
issues here that I plan to fix shortly - I've summarized them with
FIXME comments.

BUG=401504

Review URL: https://codereview.chromium.org/487663003

git-svn-id: svn://svn.chromium.org/blink/trunk@180686 bbb929c8-8fbe-4397-9dbb-9b2b20218538