Add a GrPathRange class

Adds a GrPathRange object that represents a range of paths on the gpu.
Updates GrDrawTarget::drawPaths and supporting code to use GrPathRange
instead of an array of GrPath objects.

Change-Id: I67845f3893cd4d955db947d699aa3733cbb081e0

BUG=skia:
R=bsalomon@google.com, jvanverth@google.com, kkinnunen@nvidia.com

Author: cdalton@nvidia.com

Review URL: https://codereview.chromium.org/400713003