Use GL_TEXTURE_2D for pixel readback path in GpuVideoAcceleratorFactories

Use GL_TEXTURE_2D for pixel readback path in GpuVideoAcceleratorFactories

Using the destination texture target in
GpuVideoAcceleratorFactores::ReadPixels is incorrect, as the temporary texture
should always be GL_TEXTURE_2D.

BUG=None
TEST=local build, run on CrOS snow, unittests on desktop Linux

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=225379

[sheu, 2013-09-24 19:56:16]

Tested with webgl video texture demos as well.

fischman@: PTAL

[Ami GONE FROM CHROMIUM, 2013-09-24 20:07:30]

LGTM AFAIK but this isn't very far.  How does copyTextureCHROMIUM know which
target texture_id refers to?
(I see the docs @
https://code.google.com/p/chromium/codesearch#chromium/src/gpu/GLES2/extensio...
say cTC must be given 2D for the target param; I just don't understand how it
can work with a source texture from another target).

Al: care to bless this?

[sheu, 2013-09-24 21:00:18]

AFAICT the "target" parameter to copyTextureCHROMIUM is essentially vestigial. 
The texture targets for the source and destination textures are looked up in the
command buffer decoder and validated there, independently of whatever "Ttarget"
may say (which must be GL_TEXTURE_2D).

[commit-bot: I haz the power, 2013-09-25 18:30:29]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/sheu@chromium.org/23542060/7001

[commit-bot: I haz the power, 2013-09-25 21:27:34]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/sheu@chromium.org/23542060/7001

[commit-bot: I haz the power, 2013-09-26 07:38:14]

Change committed as 225379