Make the background of the cast button semi-transparent

Make the background of the cast button semi-transparent

The overlay cast button had a semi-transparent CSS background color.
Unfortunately the button image is treated in Blink as a background
image, so this modified the image, rather than simply providing
a background for it. As a result the cast button came out as grey
on grey on pale backgrounds.

Fix this by making the CSS background color transparent, and
modifying the icon image itself to make its background semi-transparent.

Since the CSS is in Blink the CSS change is in the Blink CL
https://codereview.chromium.org/658353002

TBR=jam@chromium.org

BUG=424180
Review URL: https://codereview.chromium.org/661833005

Cr-Commit-Position: refs/heads/master@{#300130}
(cherry picked from commit fe5c903a0cb9cdf4e260fc37ac4cb4ebfe8a8cec)

Committed: https://chromium.googlesource.com/chromium/src/+/3ea3c377914eb642ed9a4fec44b4aebce00739b1

[aberent, 2014-10-22 18:31:14]

The CQ bit was checked by aberent@chromium.org

[aberent, 2014-10-22 18:33:51]

The CQ bit was unchecked by aberent@chromium.org

[commit-bot: I haz the power, 2014-10-22 18:34:03]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/674583002/1

[aberent, 2014-10-22 18:39:15]

Committed patchset #1 (id:1) manually as
3ea3c377914eb642ed9a4fec44b4aebce00739b1 (presubmit successful).