Revert 94743 - Allow the renderer process to map textures from one context into another.

Revert 94743 - Allow the renderer process to map textures from one context into another.

This is on an individual resource basis rather than general share groups in order to hide be able to hide the full namespace from untrusted plugins.

Accelerated 2D canvas now no longer needs to copy its backing store on every page composite and the redundant backing texture is not needed, saving video memory.

Unit tests for the GPU service to follow.

Patch to delete copyTextureToParentTexture extensions from gpu/ to follow.

BUG=90714
Review URL: http://codereview.chromium.org/7529015

TBR=apatrick@chromium.org
Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=94808