When uniform's value had been updated, set dirty and
update its content on D3D backend.

Bug: angleproject:2814
TEST=angle_end2end_tests.ComputeShaderTest
     .UniformDirty/ES3_1_D3D11
dEQP.GLES31/functional_image_load_store_cube*
dEQP.GLES31/functional_image_load_store_3d*
dEQP.GLES31/functional_image_load_store_2d_array*

Change-Id: Ic135c140559925fe33790475d03dc608afa92384
Reviewed-on: https://chromium-review.googlesource.com/c/1387967Reviewed-by: Jiajia Qin <jiajia.qin@intel.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Xinghua Cao <xinghua.cao@intel.com>