Resizing the diff viewer was not very smooth, and some UI elements would
stutter. The active chunk highlighter would sort of jump around a little
bit or lag behind when the chunks changed position or height. There was
also a horizontal scrollbar that would flicker on and off when resizing
the window smaller. Along with this, it was just a bit too slow to
resize.

Many of these problems were ordering issues in event processing and
style issues. We also just had some less-than-optimal logic in resize
handlers.

Resize handling is now driven primarily by the diff viewer page,
allowing some coordination to take place. Upon receiving a resize event
(or a layout is otherwise scheduled), the new _.throttleLayout() is
called. This makes use of requestAnimationFrame() to schedule the
function used to compute the new layout. If there are subsequent calls
to this before the new layout has been computed (which can happen during
resizes with very heavy pages), they'll be ignored, keeping us from
swamping the browser.

The layout handler tells each diff to update its layout, and then tells
the chunk highlighter to update. This ensures that any layout changes
from the diffs can be immediately reflected in the highlighter, and
prevents all this from constantly happening while resizing the page
(which caused resizes to be slow and events to accumulate).

The chunk highlighter's been optimized quite a bit. Some jQuery calls,
which are more expensive than needed for this work, have been replaced
with element attribute lookups, and the logic for what CSS to apply and
when has been reworked. The width is no longer set on resize. Instead,
we simply dock to the left and right of the container, allowing the
browser to handle that and preventing the horizontal scrollbars from
appearing.

Resize handlers are also more aware of vertical vs. horizontal window
sizes, preventing any work from occurring if the resize wouldn't result
in any changes.

Now that the orders are guaranteed, we can also remove the defer() for
the chunk highlighter, which allows the highlighter to smoothly appear
bound to the chunk as its position or height changes.

The result is much smoother resizing, improved logic, and simplified
code that's easier to reason about.

Tested on Chrome, Firefox, and Safari. Resizing was noticeably improved

on all browsers, particularly on Firefox and Safari.
Profiled resizing diffs and switching chunks before and after.
Unit tests pass.