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/193620Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>