Create Read-only Base class for GrDrawState that holds data members and getters

Base class will but used also in follow up cl that will create an OptDrawState class which will
share much of the same data/functions as DrawState. Thus the base class
BUG=skia:
R=bsalomon@google.com

Author: egdaniel@google.com

Review URL: https://codereview.chromium.org/506803003
5 files changed