This change fixes a bug that the image texture initialization time is not
correct. It happens when we use compute shader to write data to an
uninitialized texture, then use readPixels to read the result which
results that texture initialization falls behind image store and
covers the real result.

Bug: angleproject:2766

Change-Id: I4e986972096857afc975c40dfa4d559a2f31194c
Reviewed-on: https://chromium-review.googlesource.com/1170569
Commit-Queue: Jiajia Qin <jiajia.qin@intel.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>