CommandBufferHelper will keep a pointer to the depth-stencil
RenderTargetVk, and use this to set RenderTargetVk::mContentDefined to
true at the end of a render pass.

Test: angle_white_box_tests --gtest_filter=VulkanPerformanceCounterTest.InvalidatingAndUsingDepthDoesNotBreakRenderPass/*
Test: angle_deqp_gles3_tests --gtest_filter=dEQP.GLES3/functional_fbo_invalidate_* --use-angle=vulkan
Bug: b/163854287
Change-Id: I891381825ee01e141dfa4f9099d07d9ffc943f77
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2368194Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Charlie Lao <cclao@google.com>
Commit-Queue: Ian Elliott <ianelliott@google.com>