Make test names in the sheriff-o-matic examine page selectable

Don't use paper item. That makes things unselectable, but it also adds
the ink effect, which is weird once the text is selectable. Instead,
just use a regular old div. <core-menu> still does all the real work
of selecting items.

BUG=404043
NOTRY=true

Review URL: https://codereview.chromium.org/498653002

git-svn-id: svn://svn.chromium.org/blink/trunk@180803 bbb929c8-8fbe-4397-9dbb-9b2b20218538
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