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

BUG=424180
Committed: https://crrev.com/fe5c903a0cb9cdf4e260fc37ac4cb4ebfe8a8cec
Cr-Commit-Position: refs/heads/master@{#300130}

[aberent, 2014-10-17 11:48:14]

aberent@chromium.org changed reviewers:
+ jam@chromium.org

[jam, 2014-10-17 17:02:12]

lgtm

[aberent, 2014-10-17 17:16:23]

The CQ bit was checked by aberent@chromium.org

[commit-bot: I haz the power, 2014-10-17 17:17:11]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/661833005/1

[commit-bot: I haz the power, 2014-10-17 18:05:45]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-10-17 18:07:12]

Patchset 1 (id:??) landed as
https://crrev.com/fe5c903a0cb9cdf4e260fc37ac4cb4ebfe8a8cec
Cr-Commit-Position: refs/heads/master@{#300130}