glCopyTextureCHROMIUM calls in D3D11 would call getNativeTexture on the
copy source which would allocate a TextureStorage11 if one did not
already exist. This caused full mip-chain allocations in some cases when
we could have just copied directly from the staging Image11.

When navigating to wikipedia.org:

                      Peak memory | Stable memory
Passthrough (before):      142 mb |        119 mb
Passthrough (after):       138 mb |        106 mb
Validating:                139 mb |        110 mb

No mip-mapped textures were allocated after this change.

BUG=chromium:1030835

Change-Id: Icd14afaec445299423cfea293cba501d774f6ade
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2028147
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Rafael Cintron <rafael.cintron@microsoft.com>