Vulkan:Integrate SecondaryCommandBuffers Integrate the custom SecondaryCommandBuffer type into the CommandGraph nodes by adding new ANGLE_USE_CUSTOM_VULKAN_CMD_BUFFERS define that can be set in the BUILD gn args with angle_enable_custom_vulkan_cmd_buffers set to "true." Initially the custom cmd buffers are disabled by default. This adds some support functions to SecondaryCommandBuffer to make the integration easier by matching the wrapped cmd buffer interface: initialize(), end(), valid(). Bug: angleproject:3136 Change-Id: Ib910554583192550757bb8ce89914e3ea8737988 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1526556 Commit-Queue: Tobin Ehlis <tobine@google.com> Reviewed-by: Jamie Madill <jmadill@chromium.org>