Optimize the texture upload for texImage2D from WebGL by doing a GPU->GPU textures copy instead of with CPU readback involved.

For PerformanceTests/Canvas/upload-webgl-to-texture.html, the performance(runs/s) can have 15x ~ 20x speedup for Windows, Mac, Linux and Android.  

BUG=306497

Review URL: https://codereview.chromium.org/27005002

git-svn-id: svn://svn.chromium.org/blink/trunk@160913 bbb929c8-8fbe-4397-9dbb-9b2b20218538