Storing the color in an uniform buffer, and updating it by
vkCmdUpdateBuffer can hit a driver bug. Using vkCmdPushConstants
instead can workaround this bug. Besides, push constant is expected
to outperform memory-backed resource update according to the vkspec
(section 13.2.6).

Bug: angleproject:2547

Change-Id: I319cd8e07e0a1d0cb56005b78a46466d9fca436e
Reviewed-on: https://chromium-review.googlesource.com/1067247
Commit-Queue: Jie A Chen <jie.a.chen@intel.com>
Reviewed-by: Luc Ferron <lucferron@chromium.org>