Remove no-op blink-gc-plugin argument.

third_party/WebKit/Source/BUILD.gn

Index: third_party/WebKit/Source/BUILD.gn
diff --git a/third_party/WebKit/Source/BUILD.gn b/third_party/WebKit/Source/BUILD.gn
index b296b0e61984a31e1a93adf4e2d33e4d3d353681..25832deae57214ccc586853dd6434340a0d05e38 100644
--- a/third_party/WebKit/Source/BUILD.gn
+++ b/third_party/WebKit/Source/BUILD.gn
@@ -158,15 +158,6 @@ config("config") {
         "use-chromium-style-naming",
       ]
     }
-
-    # TODO(https://crbug.com/689864): remove once the GC plugin has been rolled
-    # with this warning default enabled.
-    cflags += [
-      "-Xclang",
-      "-plugin-arg-blink-gc-plugin",
-      "-Xclang",
-      "warn-stack-allocated-trace-method",
-    ]
   }
 }