Add drawDrawable support to GrRenderTargetContext.

This also includes adding drawable Op and plumbing it through to the GPU.

BUG=skia:

Change-Id: I0b2464c5a458c2fbf05b9528e47b9e6e3ac27d57
Reviewed-on: https://skia-review.googlesource.com/c/9645
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
diff --git a/gn/tests.gni b/gn/tests.gni
index 36dd36c..efd3418 100644
--- a/gn/tests.gni
+++ b/gn/tests.gni
@@ -283,6 +283,7 @@
   "$_tests/UtilsTest.cpp",
   "$_tests/VerticesTest.cpp",
   "$_tests/VkBackendSurfaceTest.cpp",
+  "$_tests/VkDrawableTest.cpp",
   "$_tests/VkMakeCopyPipelineTest.cpp",
   "$_tests/VkWrapTests.cpp",
   "$_tests/VptrTest.cpp",