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/PromiseImageTest.cpp b/tests/PromiseImageTest.cpp
index 7eed68f..2a43458 100644
--- a/tests/PromiseImageTest.cpp
+++ b/tests/PromiseImageTest.cpp
@@ -136,7 +136,7 @@
                                                                  expectedDoneCnt,
                                                                  reporter));
 
-        bool isVulkan = kVulkan_GrBackend == ctx->contextPriv().getBackend();
+        bool isVulkan = GrBackendApi::kVulkan == ctx->contextPriv().getBackend();
         canvas->flush();
         expectedFulfillCnt++;
         expectedReleaseCnt++;