Remove background color from overlay cast button

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.

Note that the icon image is within Chromium, so that change will be
a seperate CL.

BUG=424180

Review URL: https://codereview.chromium.org/658353002

git-svn-id: svn://svn.chromium.org/blink/trunk@184092 bbb929c8-8fbe-4397-9dbb-9b2b20218538