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/TestContext.h b/tools/gpu/TestContext.h
index 8e939a6..7a92bc0 100644
--- a/tools/gpu/TestContext.h
+++ b/tools/gpu/TestContext.h
@@ -59,7 +59,7 @@
      */
     SkScopeExit SK_WARN_UNUSED_RESULT makeCurrentAndAutoRestore() const;
 
-    virtual GrBackend backend() = 0;
+    virtual GrBackendApi backend() = 0;
 
     virtual sk_sp<GrContext> makeGrContext(const GrContextOptions&);