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1. Expose IconDescription in vector icon code so when we add or
   modify parameters we don't have to update the API.
   TODO: remove the parts of the API that use multiple parameters
         and replace them all with IconDescription.
2. Add AnimatedIconView, which caches the start and end states and
   controls animating in between as well.
3. Add an animated version of browser_tools.icon. This was crafted
   by hand based on the non-animated version and a visual description,
   but in the future designers should deliver Android vector drawables
   which will make the process of creating these icons much easier
   (and perhaps automate-able).
   TODO: actually craft a 2x version instead of reusing 1x.
4. Apply this all to the app menu. This solves the issue of pixel
   perfection in the steady state.

BUG=718549,704786

Review-Url: https://codereview.chromium.org/2892563004
Cr-Commit-Position: refs/heads/master@{#476425}