Disable -Wmemset-transposed-args when building with ToT clang

This warning has a few false positives in ffmpeg.

TBR=thakis@chromium.org
BUG=866202

Change-Id: I08634bdeabe6e30225d37941b078c540c21a74bc
Reviewed-on: https://chromium-review.googlesource.com/1145887Reviewed-by: Reid Kleckner <rnk@chromium.org>
Commit-Queue: Reid Kleckner <rnk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577055}

build/config/compiler/BUILD.gn

@@ -1482,6 +1482,9 @@ config("default_warnings") {
           # TODO(thakis): Only for no_chromium_code? http://crbug.com/505314
           "-Wno-ignored-pragma-optimize",
         ]
+        if (llvm_force_head_revision) {
+          cflags += [ "-Wno-memset-transposed-args" ]
+        }
       }
     }
   }