Fix for nexus 5 crashing in GL benches

Fix for nexus 5 crashing in GL benches

GLBenches do not expect gl state  to change between onPerCanvasPreDraw and *PostDraw, but we do a clear and sometimes we clear as draw.  This causes us to bind vertex objects / programs / etc.

This change creates two new virtual methods which are called right before and immediately after timing.

BUG=skia:

Review URL: https://codereview.chromium.org/1379853003