Specs for the animation
 - Duration: 0.3s
 - cubic-bezier(.25,.1,.25,1) http://cubic-bezier.com/#.25,.1,.25,1

i'm not actually working on this, started with  bug 788018  (which is related but not quite the same)

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/28f6dfebd737dd564b32f369a10d1c0738302d63

commit 28f6dfebd737dd564b32f369a10d1c0738302d63
Author: Dave Schuyler <dschuyler@chromium.org>
Date: Wed Nov 29 20:18:10 2017

[MD extensions] animate dev controls

This cl transitions to/from showing the dev controls (developer mode)
over time, rather than snapping in/out of view.

Bug:  787220 
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I4b7e8176a284b77a68eced3b346031986340ff72
Reviewed-on: https://chromium-review.googlesource.com/792537
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Dave Schuyler <dschuyler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520219}
[modify] https://crrev.com/28f6dfebd737dd564b32f369a10d1c0738302d63/chrome/browser/resources/md_extensions/toolbar.html
[modify] https://crrev.com/28f6dfebd737dd564b32f369a10d1c0738302d63/chrome/browser/resources/md_extensions/toolbar.js