Use different icons for the overlay and non-overlay cast buttons

Use different icons for the overlay and non-overlay cast buttons

We want the overlay cast icon to be visible on all backgrounds, so its
background has to be at least semi-opaque. The non-overlay cast icon,
however, should have the same background as the media controls, which
is easiest to achive by making its background transparent. As such we
need different icons for the two cases.

This CL simply provides the new icons. The corresponding Blink CL, which
uses them, is https://codereview.chromium.org/716613002.

BUG=426373
Review URL: https://codereview.chromium.org/711083002

TBR=jam@chromium.org

Cr-Commit-Position: refs/heads/master@{#303620}
(cherry picked from commit bea7c9b7f0a387378fd2756430712e6042ce2c2c)

Committed: https://chromium.googlesource.com/chromium/src/+/3605314e7ebea5381835cd2d3cac078496214de1

[aberent, 2014-11-13 10:54:30]

Committed patchset #1 (id:1) manually as
3605314e7ebea5381835cd2d3cac078496214de1 (presubmit successful).