When storage buffer bound is unchanged and shader writes it,
buffer content should also be updated. Currently, this case
cannot work normally on D3D backend.

Bug: angleproject:2814
TEST=angle_end2end_tests.ComputeShaderTest.StorageBufferBoundUnchanged

Change-Id: I6f25b7eac84c44392befaca61a33bdf2457f29f9
Reviewed-on: https://chromium-review.googlesource.com/c/1391880
Commit-Queue: Xinghua Cao <xinghua.cao@intel.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>