add test cases for drawing dynamic WebGL to multiple 2d Canvases.

This test is added to test the correctness of drawing dynamic WebGL to multiple 2d Canvases. Specially, it is to verify the issue of http://code.google.com/p/chromium/issues/detail?id=233205 to check if the two mutable immediate storage is handled correctly in a dynamic sense and won't be overwrote.