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

Review URL: https://codereview.chromium.org/23542060

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225379 0039d316-1c4b-4281-b951-d872f2087c98