Bug 1665455 - Animate the Skeleton UI r=agashlin

For a better user experience during slow startups (and to match the design
by Markus Jaritz), we want to animate the placeholder elements (the grey
rectangles which hold the place of text and icons) with a light moving
gradient.

A question may arise regarding whether the performance cost of running this
animation is worth the improved user experience. My claim is yes, hinging
mostly on the observation that the performance cost is small.

On my machine, one frame of the animation takes at most 0.15ms, and runs
every 16.15ms. This means we eat less than 1% CPU time on one core of the
system. It should also be noted that this animation runs primarily during
the window wherein we are prefetching dlls, AKA while we are blocked on IO
and the CPU is more likely to be idle.

On slower systems, one frame may take longer - however, on slower systems
we should also be blocked *more* by IO, making the trade favorable.

For further anecdotal evidence of this being okay, when I run this on slow
reference hardware shortly after OS startup, I do not see any dropped frames,
indicating that this has very little trouble completing, and is thus quite
cheap.

Regarding testing, I will invoke the same logic as for the rest of the
skeleton UI patches - it would involve quite a bit of work to test this in
automation given our existing frameworks. It may be worth it at some point,
but certainly not while this is a feature that we are just experimenting
with and which is not enabled by default.

Differential Revision: https://phabricator.services.mozilla.com/D91453