Add a subtle reminder that chrome_extensions.js is overwritten.

third_party/closure_compiler/bump_compiler_version

Index: third_party/closure_compiler/bump_compiler_version
diff --git a/third_party/closure_compiler/bump_compiler_version b/third_party/closure_compiler/bump_compiler_version
index 15e802b749231337b1a288a3f20eca57fbb3cfa4..c481b06c724383787afdd1489a152a63ed25cee9 100755
--- a/third_party/closure_compiler/bump_compiler_version
+++ b/third_party/closure_compiler/bump_compiler_version
@@ -32,7 +32,24 @@ fi
 
 echo "Copying compiler.jar and chrome_extensions.js"
 cp build/compiler.jar "${SCRIPT_DIR}/compiler/"
-cp contrib/externs/chrome_extensions.js "${SCRIPT_DIR}/externs/"
+(cat <<EOT && cat contrib/externs/chrome_extensions.js) > "${SCRIPT_DIR}/externs/chrome_extensions.js"
+//    SSSSSSSSSSSSSSS TTTTTTTTTTTTTTTTTTTTTTT     OOOOOOOOO     PPPPPPPPPPPPPPPPP
+//  SS:::::::::::::::ST:::::::::::::::::::::T   OO:::::::::OO   P::::::::::::::::P
+// S:::::SSSSSS::::::ST:::::::::::::::::::::T OO:::::::::::::OO P::::::PPPPPP:::::P
+// S:::::S     SSSSSSST:::::TT:::::::TT:::::TO:::::::OOO:::::::OPP:::::P     P:::::P
+// S:::::S            TTTTTT  T:::::T  TTTTTTO::::::O   O::::::O  P::::P     P:::::P
+// S:::::S                    T:::::T        O:::::O     O:::::O  P::::P     P:::::P
+//  S::::SSSS                                                     P::::PPPPPP:::::P
+//   SS::::::SSSSS       This file is generated. To update it,    P:::::::::::::PP
+//     SSS::::::::SS          run bump_compiler_version.          P::::PPPPPPPPP
+//        SSSSSS::::S                                             P::::P
+//             S:::::S        T:::::T        O:::::O     O:::::O  P::::P
+//             S:::::S        T:::::T        O::::::O   O::::::O  P::::P
+// SSSSSSS     S:::::S      TT:::::::TT      O:::::::OOO:::::::OPP::::::PP
+// S::::::SSSSSS:::::S      T:::::::::T       OO:::::::::::::OO P::::::::P
+// S:::::::::::::::SS       T:::::::::T         OO:::::::::OO   P::::::::P
+//  SSSSSSSSSSSSSSS         TTTTTTTTTTT           OOOOOOOOO     PPPPPPPPPP
+EOT
 
 echo "Rebuilding runner.jar"
 "${SCRIPT_DIR}/runner/build_runner_jar.py"