Add ability to write out VkPipelineCache to gpu PersistentCache.

Bug: skia:
Change-Id: Id13d680401f69a074ae0c85f9ceaf3308fccb129
Reviewed-on: https://skia-review.googlesource.com/c/181403
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
diff --git a/src/gpu/vk/GrVkPipelineStateBuilder.h b/src/gpu/vk/GrVkPipelineStateBuilder.h
index fc322df..7351789 100644
--- a/src/gpu/vk/GrVkPipelineStateBuilder.h
+++ b/src/gpu/vk/GrVkPipelineStateBuilder.h
@@ -44,7 +44,11 @@
                           GrPrimitiveType primitiveType,
                           GrVkGpu* gpu);
 
+        size_t shaderKeyLength() const { return fShaderKeyLength; }
+
     private:
+        size_t fShaderKeyLength;
+
         typedef GrProgramDesc INHERITED;
     };