Cleanup DrawingBuffer::paintCompositingResultsToCanvas

Remove the unnecessary copyTexture and use the value tracked by 
WebGLRenderingContext when restoring the framebuffer binding instead of
querying GL_FRAMEBUFFER_BINDING.

BUG=

Review URL: https://codereview.chromium.org/409483002

git-svn-id: svn://svn.chromium.org/blink/trunk@178723 bbb929c8-8fbe-4397-9dbb-9b2b20218538