Move GrBackend enum to enum class.

Bug: skia:
Change-Id: I8add2470313549723be6720b543091c444d2eb4f
Reviewed-on: https://skia-review.googlesource.com/c/161680
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
diff --git a/src/gpu/vk/GrVkGpu.cpp b/src/gpu/vk/GrVkGpu.cpp
index 371fd31..d002955 100644
--- a/src/gpu/vk/GrVkGpu.cpp
+++ b/src/gpu/vk/GrVkGpu.cpp
@@ -1473,7 +1473,7 @@
 }
 
 bool GrVkGpu::isTestingOnlyBackendTexture(const GrBackendTexture& tex) const {
-    SkASSERT(kVulkan_GrBackend == tex.fBackend);
+    SkASSERT(GrBackendApi::kVulkan == tex.fBackend);
 
     GrVkImageInfo backend;
     if (!tex.getVkImageInfo(&backend)) {
@@ -1495,7 +1495,7 @@
 }
 
 void GrVkGpu::deleteTestingOnlyBackendTexture(const GrBackendTexture& tex) {
-    SkASSERT(kVulkan_GrBackend == tex.fBackend);
+    SkASSERT(GrBackendApi::kVulkan == tex.fBackend);
 
     GrVkImageInfo info;
     if (tex.getVkImageInfo(&info)) {
@@ -1526,7 +1526,7 @@
 }
 
 void GrVkGpu::deleteTestingOnlyBackendRenderTarget(const GrBackendRenderTarget& rt) {
-    SkASSERT(kVulkan_GrBackend == rt.fBackend);
+    SkASSERT(GrBackendApi::kVulkan == rt.fBackend);
 
     GrVkImageInfo info;
     if (rt.getVkImageInfo(&info)) {