Dawn: fix MSC build.

Only pass -Wno-microsoft-cast for clang.

Change-Id: I10b610784bbbcfb397f5249f5050f1d1b0a0b69c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/293345
Reviewed-by: Weston Tracey <westont@google.com>
Commit-Queue: Stephen White <senorblanco@chromium.org>
diff --git a/BUILD.gn b/BUILD.gn
index 8e924f8..1465e8e 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -1407,7 +1407,9 @@
     if (skia_use_dawn) {
       public_deps += [ "//third_party/externals/dawn/src/dawn:dawn_headers" ]
       sources += [ "tools/gpu/dawn/DawnTestContext.cpp" ]
-      cflags_cc = [ "-Wno-microsoft-cast" ]
+      if (is_clang) {
+        cflags_cc = [ "-Wno-microsoft-cast" ]
+      }
     }
 
     if (is_fuchsia && using_fuchsia_sdk) {