On Views platforms, I could just barely perceive the animation, and it
added jank, not smoothness, to the transition. On Mac, the download
shelf animated in/out *after* the fullscreen transition, which was odd
and drew attention to it as it disappeared.

Now, the shelf appears/disappears as part of the transition — so on
Views it doesn't add jank, and on Mac it fades in/out along with the
rest of the window.

As an added bonus, this fixes a bug where windows on Mac grew by the
height of the downloads bar each time you enter/exit fullscreen. It
should also fix a flaky test on 10.9 by adding an option to hide/show
the shelf without animation.

BUG=29629, 687447

Review-Url: https://codereview.chromium.org/2688413012
Cr-Commit-Position: refs/heads/master@{#451327}