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/tools/gpu/mock/MockTestContext.cpp b/tools/gpu/mock/MockTestContext.cpp
index 6075bc8..3bcf0f1 100644
--- a/tools/gpu/mock/MockTestContext.cpp
+++ b/tools/gpu/mock/MockTestContext.cpp
@@ -19,7 +19,7 @@
     MockTestContext() {}
     ~MockTestContext() override {}
 
-    virtual GrBackend backend() override { return kMock_GrBackend; }
+    virtual GrBackendApi backend() override { return GrBackendApi::kMock; }
 
     void testAbandon() override {}
     void submit() override {}