Composited images should respond to dynamic style changes

Directly composited images can exit direct composition if style updates
demand that, but render incorrectly. Reset the contents image to 0, add
layout tests (issue 405803, issue 408460).

  compositing/images/direct-image-dynamic-border-draws-content.html
  compositing/images/direct-image-dynamic-border-radius.html
  compositing/images/direct-image-dynamic-clip-path.html

Add imageLayerContentReset to unit test to test setting and resetting a
graphics layer image with setContentToImage(), and that the changes are
reported by the graphics layer hasContentsLayer() and contentsLayer().

BUG=405803, 408460

Review URL: https://codereview.chromium.org/500493003

git-svn-id: svn://svn.chromium.org/blink/trunk@181154 bbb929c8-8fbe-4397-9dbb-9b2b20218538