The diff file index in the Unified Banner only updated extents when new

diffs were added, when going into docked mode, or on window resize. If a

diff was expanded or collapsed, the diff file index didn't know about

this, and could present the wrong entries for the current view.
We now use a ResizeObserver to track resizes to all the diffs,

updating the extents whenever a resize is detected.
To avoid duplicate calls to updateLayout() where this is computed

(which could occur due to a window resize alone, or a window resize

along with a ResizeObserver event), we now call a

queueUpdateLayout() function, which will use requestAnimationFrame()

to schedule updateLayout(). This, along with storing a request ID,

prevents us from doing any unnecessary work that could lead to sluggish

resizes.

Unit tests pass.
Tested expanding and collapsing diffs, resizing the window, and manually

altering the DOM, verifying that layout updates were always called and

the diff file index always showed the correct result.