More genericity: overload isPaintOpaque(SkPaint, SkBitmap)

Instead of taking a Bitmap as an argument this version
takes only the type of content (None/Opaque/Transparent).
This will be used to check the opaqueness of a SkPaint
that draws a SkImage.

BUG=skia:3042

Review URL: https://codereview.chromium.org/663233002