This sets ContextVk::mRenderPassCommandBuffer properly when glClear get
called so that it wonr end up start anotyer render pass during setupDraw
call.

Bug: angleproject:4395
Change-Id: Id2110719ad4d70b4c410062466d32381fb5e1f88
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2050966
Commit-Queue: Charlie Lao <cclao@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>