[Extensions] Make runtime.onMessageExternal unmanaged

All messaging events (runtime.onMessage, runtime.onMessageExternal,
extension.onRequest, extension.onRequestExternal) are handled directly
by custom messaging bindings, rather than by the EventRouter. However,
runtime.onMessageExternal wasn't marked as unmanaged in the schema. Fix
this.

Bug: None
Change-Id: I438b273c4046f91367fd44e9fcfd8a537ae7f13e
Reviewed-on: https://chromium-review.googlesource.com/786899Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519866}

extensions/common/api/runtime.json

@@ -541,6 +541,7 @@
         "name": "onMessageExternal",
         "type": "function",
         "nocompile": true,
+        "options": { "unmanaged": true },
         "description": "Fired when a message is sent from another extension/app (by $(ref:runtime.sendMessage)). Cannot be used in a content script.",
         "parameters": [
           {"name": "message", "type": "any", "optional": true, "description": "The message sent by the calling script."},