Add initial support for creating a vulkan secondary command buffer drawing context.

This sets up the context and adds support for creating RTContexts, RTProxies, RTs,
and GrVkRenderPass's that wrap the external secondary command buffer.

Bug: skia:
Change-Id: I80ebbb690a5fe464f775c5fcad651dfe2a150418
Reviewed-on: https://skia-review.googlesource.com/c/178926
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
diff --git a/gn/gpu.gni b/gn/gpu.gni
index d97edbe..a8a0602 100644
--- a/gn/gpu.gni
+++ b/gn/gpu.gni
@@ -572,6 +572,7 @@
   "$_include/gpu/vk/GrVkBackendContext.h",
   "$_include/gpu/vk/GrVkExtensions.h",
   "$_include/gpu/vk/GrVkMemoryAllocator.h",
+  "$_include/gpu/vk/GrVkSecondaryCBDrawContext.h",
   "$_include/gpu/vk/GrVkTypes.h",
   "$_include/gpu/vk/GrVkVulkan.h",
   "$_include/private/GrVkTypesPriv.h",
@@ -637,6 +638,7 @@
   "$_src/gpu/vk/GrVkSampler.h",
   "$_src/gpu/vk/GrVkSamplerYcbcrConversion.cpp",
   "$_src/gpu/vk/GrVkSamplerYcbcrConversion.h",
+  "$_src/gpu/vk/GrVkSecondaryCBDrawContext.cpp",
   "$_src/gpu/vk/GrVkSemaphore.cpp",
   "$_src/gpu/vk/GrVkSemaphore.h",
   "$_src/gpu/vk/GrVkStencilAttachment.cpp",