Introduces pushDebugGroup, popDebugGroup, and insertDebugMarker implementations
for Vulkan and Metal using VK_EXT_debug_marker and XCode, respectively.
Bug: dawn:44
Change-Id: I0ae56c4d67aa832123f27a1fcdddf65746261e57
Reviewed-on: https://dawn-review.googlesource.com/c/4241
Commit-Queue: Brandon Jones <brandon1.jones@intel.com>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>