Fix some TODO(GYP) in the GN build.

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 1f316a415d7bde43c25adaf7be63eb979bf0977c..6ecca3e43d562ea0ea3ab442cef3b634150e2a46 100644
--- a/third_party/WebKit/Source/BUILD.gn
+++ b/third_party/WebKit/Source/BUILD.gn
@@ -81,11 +81,6 @@ config("config") {
       "blink-gc-plugin",
     ]
   }
-
-  ## TODO(GYP) : gn does not yet support use_system_icu.

[brettw, 2016/05/23 22:06:26]

use_system_icu isn't required, and if we add it it should be integrated
differently than this.

-  #if (use_system_icu) {
-  #  defines += [ "USING_SYSTEM_ICU" ]
-  #}
 }
 
 # The follow configs apply to all targets except for unit tests, which rely on