Use MediaValuesDynamic in HTMLImageElement rather than MediaValuesCached

This is a speculative fix to performance issues introduced by the sizes impl.
Since there's no reason to use MediaValuesCached in HTMLImageElement,
and since it may have an overhead of caching all the media values,
I've switched HTMLImageElement to use MediaValuesDynamic.

In order to avoid problems when the LocalFrame doesn't exist or doesn't have a view,
I've added a MediaValuesDynamic::create() method that return an empty MediaValuesCached
n these cases.

BUG=389616

Review URL: https://codereview.chromium.org/357153002

git-svn-id: svn://svn.chromium.org/blink/trunk@177252 bbb929c8-8fbe-4397-9dbb-9b2b20218538