Diff fragment expansion controls will only expand if the mouse hovers

over the diff fragment for a half of a second. This unifies behaviour

in Firefox in Chrome so that when you scroll down a review request,

the diff fragment controls won't expand as you scroll by.
Scrolling to a diff fragment and stopping now works correctly in both

Firefox and Chrome. That is, the diff fragment will expand as long as

the mouse stays it for half of a second.
The diffFragmentQueueView now maintains the number of mouse watchers

so that multiple mouse move event handlers are not added and the event

handler is only removed once there are no active mouse watchers. This

behaviour is used in the case of multiple diff fragment expansion

events and/or scrolling over diff fragments occuring near the same

time.

Verified in Firefox, Chrome, and Safari.