Revert 131177 - aura: Change shared context to be offscreen for arm

Revert 131177 - aura: Change shared context to be offscreen for arm

This change causes Chrome OS to get stuck in the login screen on Lumpy and Stumpy, so it is reverted for now.

BUG=chrome-os-partner:8831
TEST=Manually checked that I can log in successfully

ARM drivers don't like to have 2 EGLSurface on the same Window. So we need to
make the UI shared context offscren.
This also requires the image transport surface to be an "offscreen" GLSurface (a
pbuffer), however the GLES2Decoder needs to be "onscreen" (i.e. it calls
SwapBuffers on the surface instead managing the offscreen buffers itself).
So this decouples the 2 notions of "offscreen".

BUG=chrome-os-partner:8284
TEST=gpu_unittests. Aura chrome --ui-use-gpu-process with poster circle and webgl on tegra, lumpy, desktop GL. Desktop chrome with poster circle and webgl.


Review URL: http://codereview.chromium.org/10007034

TBR=piman@chromium.org
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=131559