Add new GrVkBackendContext::Create explicitly requiring vk proc getters

Also remove the feature of GrVkGpu that creates the instance/device if the client doesn't provide one.

Change-Id: Ie617313b6c684ed355333a475b80d0aae7e3a026
Reviewed-on: https://skia-review.googlesource.com/14261
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
diff --git a/src/gpu/vk/GrVkGpu.cpp b/src/gpu/vk/GrVkGpu.cpp
index 3d30e35..d8f1037 100644
--- a/src/gpu/vk/GrVkGpu.cpp
+++ b/src/gpu/vk/GrVkGpu.cpp
@@ -77,10 +77,7 @@
     const GrVkBackendContext* vkBackendContext =
         reinterpret_cast<const GrVkBackendContext*>(backendContext);
     if (!vkBackendContext) {
-        vkBackendContext = GrVkBackendContext::Create();
-        if (!vkBackendContext) {
-            return nullptr;
-        }
+        return nullptr;
     } else {
         vkBackendContext->ref();
     }