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/gl/GLTestContext.h b/tools/gpu/gl/GLTestContext.h
index f5a0593..d376ed6 100644
--- a/tools/gpu/gl/GLTestContext.h
+++ b/tools/gpu/gl/GLTestContext.h
@@ -20,7 +20,7 @@
 public:
     ~GLTestContext() override;
 
-    virtual GrBackend backend() override { return kOpenGL_GrBackend; }
+    virtual GrBackendApi backend() override { return GrBackendApi::kOpenGL; }
 
     bool isValid() const { return SkToBool(this->gl()); }