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/tests/ProxyTest.cpp b/tests/ProxyTest.cpp
index 5e050a3..8988ed5 100644
--- a/tests/ProxyTest.cpp
+++ b/tests/ProxyTest.cpp
@@ -234,7 +234,7 @@
 
                 // Test wrapping FBO 0 (with made up properties). This tests sample count and the
                 // special case where FBO 0 doesn't support window rectangles.
-                if (kOpenGL_GrBackend == ctxInfo.backend()) {
+                if (GrBackendApi::kOpenGL == ctxInfo.backend()) {
                     GrGLFramebufferInfo fboInfo;
                     fboInfo.fFBOID = 0;
                     static constexpr int kStencilBits = 8;