Fixed Shadow blur for transparent objects

Added image filter as another option to drawLooper for rendering shadows, when alpha must be respected.

BUG=100703

Review URL: https://codereview.chromium.org/669123002

git-svn-id: svn://svn.chromium.org/blink/trunk@184443 bbb929c8-8fbe-4397-9dbb-9b2b20218538