add test cases for drawing the same webgl content to multiple Canvases

Add test cases for drawing the same webgl content to multiple Canvases, including HW accelerated 2D canvas and SW Canvases. 

It is used to verify the issue at https://codereview.chromium.org/107903003/ to check if it gets the right result and optimal performance when drawing the same WebGL content to HW accelerated Canvas and SW Canvases at the same time.  It should draw Texture based SkBitmap for HW accelerated Canvas case and draw cached SkBitmap for multiple SW Canvases.