Replace CHROMIUM_image usage with GL_CHROMIUM_gpu_memory_buffer_image.

This is now the preferred way to create an image when it
doesn't need to be mapped as the implementation can chose
whatever pixel format is ideal with for the current platform
and driver.

This removes the API that allows buffers to be mapped. If
necessary, we can re-introduce this API later in its new form
once the refactor that allow buffers to be created and mapped
on worker threads has landed.

BUG=418553
TBR=jamesr@chromium.org

Review URL: https://codereview.chromium.org/600623003

git-svn-id: svn://svn.chromium.org/blink/trunk@183224 bbb929c8-8fbe-4397-9dbb-9b2b20218538