add alternative to isRect named asRect

This defines yunchao's proposed interface in terms of
an existing implementation.

BUG=skia:
R=reed@google.com, yunchao.he@intel.com

Author: caryclark@google.com

Review URL: https://codereview.chromium.org/140953003

git-svn-id: http://skia.googlecode.com/svn/trunk@13183 2bbb7eff-a529-9590-31e7-b0007b416f81
3 files changed