Zoom bubble: Use white on blue text on hover

In the zoom bubble, when hovering over a button the background changes
to blue but the text remains gray. This CL changes the text color to
white on hover.

BUG= 297807
TEST=Zoomed into a page. Verified that button text changes to white
on hover.
NOTRY=true

Review URL: https://chromiumcodereview.appspot.com/24238010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225187 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225187