Fix a bug in Image::imageForDefaultFrame() when image anim. is finished

Image::imageForDefaultFrame() is supposed to return the poster frame
of an animated image (or the first frame) but it was returning the
current frame when called on an image for which the animation is
finished (the laest frame then).

This, because the Image::maybeAnimated() method returns false when the
animation is finished.

A new method isAnimated() has been added that only checks the number
of frames in the image to decide if the image is animated or not.

BUG=286673,405161

Review URL: https://codereview.chromium.org/474353002

git-svn-id: svn://svn.chromium.org/blink/trunk@180804 bbb929c8-8fbe-4397-9dbb-9b2b20218538