Followup 2fcc4c0: fix bugs with selection-only polling

The optimizations in selection-only polling were too aggressive.
When the cursor moved to a different node and the next poll was
a selection-only poll, we would not update this.node because that
code path was skipped for selection-only polls. Changing the
selection by clicking with the mouse will cause both a
selection-only poll (from onDocumentSelectionChange) and
a full poll (from a timeout in onDocumentMouseDown) to occur,
but they can occur in either order, at least in Chrome.
Probably what's going on is that there may or may not be
a timer tick between mousedown and selectionchange, depending
on how fast the user clicks or something.

I think this is related to recent cases I've seen of inserted
characters at the end of a line ending up on the next line.
I would imagine something like this could have caused that:
* User clicks at end of line
* mousedown happens, sets timeout for full poll
* DOM selectionchange event happens, does selectionOnly poll
* Poll emits selectionchange event but does not update this.node
* Timeout fires, full poll happens
* Poll observes that DOM selection hasn't changed, does nothing
* User types 'A', causes full poll
* Poll observes this.node has changed, does not check for content change
* No contentChange emitted, 'A' not added to DM, DM and CE out of sync
* Subsequent polls do nothing because DOM selection didn't change
* User types again
* Poll observes 'B' was added, emits contentChange
* CE sees <p>HelloA|B|</p>, computes insertion offset 7
* DM doesn't have 'A', so "insert B at 7" is <p>Hello</p>|B|
* This is normalized to <p>Hello</p><p>B</p> and committed
* New paragraph is rendered but old paragraph isn't rerendered
* CE DOM now shows <p>HelloAB</p><p>B</p>
* User is confused and goes to Roan's desk

Change-Id: I6333590e46c6c7241b8974afe5e5dfed9d79c877
(cherry picked from commit fdbbad7f29e1159494c0573dd1866a6f2f62d8a1)