Refactor Renderer11 to accomodate pixel pack. Splitting the readTextureData call allow a pack buffer read to return asynchronously after the D3D11 CopySubresource call. Refactoring patch only. BUG=angle:511 Change-Id: I3459ddfa7b48497b25fb2f4890e1cc1341e2cf75 Reviewed-on: https://chromium-review.googlesource.com/193620 Reviewed-by: Geoff Lang <geofflang@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org>