Makes blending methods in WebLayer.h pure virtuals. Adds a layout test.

- makes blendMode() and isRootForIsolatedGroup() pure virtuals
  (the Chromium code does implement these now)
- adds a layout reftest testing clipping layers with blending. This
  reftest has been reviewed with http://crrev.com/23511004/, but it
  couldn't pass because compositor-renderer bindings were
  not submitted yet.

BUG=243223

Review URL: https://codereview.chromium.org/103123002

git-svn-id: svn://svn.chromium.org/blink/trunk@163158 bbb929c8-8fbe-4397-9dbb-9b2b20218538