Take the following situation:

1. Start RP with D/S undefined: loadOp = DONT_CARE, storeOp = STORE
  * At this point, onDepthStencilWrite calls image->onWrite, setting
    depth/stencil contents defined.
2. At endRP, observe depth/stencil is not used: storeOp = DONT_CARE
3. Start another RP with D/S: loadOp = LOAD, storeOp = STORE

Because the call to image->onWrite was done at startRP, the contents of
the depth/stencil image is marked as defined, and the next render pass
is loading these data.  This change moves image->onWrite to
endRenderPass, and only calls it if storeOp = STORE, taking advantage of
all the opportunistic optimizations that try to set storeOp to another
value.

Bug: angleproject:4836
Change-Id: I9858e5caa6b1f67f841a5c6356e66927356ef469
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2548319Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Charlie Lao <cclao@google.com>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>