No need to fix-up alpha channel for text.

Now we always use skia for painting text, which unlike GDI properly handles alpha channel.

BUG=230853

Review URL: https://chromiumcodereview.appspot.com/21105005

git-svn-id: svn://svn.chromium.org/blink/trunk@155300 bbb929c8-8fbe-4397-9dbb-9b2b20218538