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

This reverts commit 36aabc8e8d4034d1cb08a0bc276e8280583319d9.

This breaks compilation with ToT (the chromium side has been reverted
    in https://codereview.chromium.org/105333002)

TBR=tonyg@chromium.org,rosca@adobe.com
BUG=None

Review URL: https://codereview.chromium.org/104653006

git-svn-id: svn://svn.chromium.org/blink/trunk@163187 bbb929c8-8fbe-4397-9dbb-9b2b20218538