Goal: ensure we always balance lock/unlock pixels calls.

Goal: ensure we always balance lock/unlock pixels calls.

A big caller of lockPixels is setContext in the bitmapshader.

This change replaces beginSession/endSession with adding endContext(), and
adds debugging code to ensure that
1. setContext calls are never nested
2. endContext is always called after each setContext call.
Review URL: https://codereview.appspot.com/6937046

git-svn-id: http://skia.googlecode.com/svn/trunk@6798 2bbb7eff-a529-9590-31e7-b0007b416f81