Add missing gyp dependency for timeline_message_handler.js.

runtime/observatory/observatory_sources.gypi

Index: runtime/observatory/observatory_sources.gypi
diff --git a/runtime/observatory/observatory_sources.gypi b/runtime/observatory/observatory_sources.gypi
index a3a2510a90d05fc25a3c6b1e32f47fc0e3287482..9c9baec63fb1d2464b8248176da6f4b365414e88 100644
--- a/runtime/observatory/observatory_sources.gypi
+++ b/runtime/observatory/observatory_sources.gypi
@@ -14,8 +14,8 @@
     'lib/object_graph.dart',
     'lib/service.dart',
     'lib/service_common.dart',
-    'lib/service_io.dart',
     'lib/service_html.dart',
+    'lib/service_io.dart',
     'lib/src/app/analytics.dart',
     'lib/src/app/application.dart',
     'lib/src/app/location_manager.dart',
@@ -45,6 +45,7 @@
     'lib/src/elements/context_view.html',
     'lib/src/elements/cpu_profile.dart',
     'lib/src/elements/cpu_profile.html',
+    'lib/src/elements/css/shared.css',
     'lib/src/elements/curly_block.dart',
     'lib/src/elements/curly_block.html',
     'lib/src/elements/debugger.dart',
@@ -77,6 +78,9 @@
     'lib/src/elements/heap_snapshot.html',
     'lib/src/elements/icdata_view.dart',
     'lib/src/elements/icdata_view.html',
+    'lib/src/elements/img/chromium_icon.png',
+    'lib/src/elements/img/dart_icon.png',
+    'lib/src/elements/img/isolate_icon.png',
     'lib/src/elements/inbound_reference.dart',
     'lib/src/elements/inbound_reference.html',
     'lib/src/elements/instance_ref.dart',
@@ -145,18 +149,15 @@
     'lib/src/elements/vm_ref.html',
     'lib/src/elements/vm_view.dart',
     'lib/src/elements/vm_view.html',
-    'lib/src/elements/css/shared.css',
-    'lib/src/elements/img/chromium_icon.png',
-    'lib/src/elements/img/dart_icon.png',
-    'lib/src/elements/img/isolate_icon.png',
     'lib/src/service/object.dart',
     'lib/tracer.dart',
     'lib/utils.dart',
+    'web/favicon.ico',
     'web/index.html',
     'web/main.dart',
-    'web/favicon.ico',
     'web/third_party/trace_viewer_full.html',
-    'web/timeline.js',
     'web/timeline.html',
+    'web/timeline.js',
+    'web/timeline_message_handler.js',
   ],
 }