Replace NativeImageSkia usage with SkBitmap in ImageFrame

Make ImageFrame store an SkBitmap directly instead of indirectly via
NativeImageSkia.
The direct effect is less indirections in ImageFrame, but this allows
making NativeImageSkia more immutable-looking since the SkBitmap&
accessor and setDataComplete() method can be removed.

Review URL: https://codereview.chromium.org/170463002

git-svn-id: svn://svn.chromium.org/blink/trunk@170553 bbb929c8-8fbe-4397-9dbb-9b2b20218538