Fix RenderImage invalidation under zoom

This patch changes the coordinate system used by RenderImage for
invalidation rects to account for zoom. When an SVG image issues
invalidations to its ImageResource clients, the invalidation rect does
not contain zoom. This is due to how SVG images work internally:
images are laid out without zoom so that relative sizes are resolved
correctly; as a result the invalidation rects do not include zoom.
This patch simply removes the zoom from the image size in
RenderImage::repaintOrMarkForLayout and clarifies the comment.

This change should not affect bitmap image invalidation because
bitmaps (even animated gifs) do not use invalidation rects.

BUG=375765

Review URL: https://codereview.chromium.org/294403003

git-svn-id: svn://svn.chromium.org/blink/trunk@174874 bbb929c8-8fbe-4397-9dbb-9b2b20218538