Move sampleCount into the DrawingBuffer class

Instead of querying it all the time, we move it into the class.
That way we will be able to reuse it in more places.

BUG=322706

Review URL: https://codereview.chromium.org/111113005

git-svn-id: svn://svn.chromium.org/blink/trunk@164264 bbb929c8-8fbe-4397-9dbb-9b2b20218538