This change book-ends RenderPasses in the Vulkan back-end with
timestamp queries. This allows us to write out a trace of GPU/CPU that
we can inspect in Chrome. To enable the GPU trace you can define
ANGLE_ENABLE_VULKAN_GPU_TRACE_EVENTS in ContextVk.cpp and run the
angle_perftests suite with --enable-trace.

Note that we ran into some issues with the implementation not returning
monotonic timestamps. This may be a driver bug but bears more
investigation.

Bug: angleproject:4433
Change-Id: I0e6a364367f15183068e55686549cb418aa94c2a
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2081380Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>