[Extensions] Remove an obsolete TODO

The TODO called for testing for action.onClicked, which exists in the
form of MultiActionAPITest.OnClickedDispatching.

Bug: 893373
Change-Id: Id33a00340aebcbc11be1d8f15b98663a662bcb89
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2288496Reviewed-by: David Bertoni <dbertoni@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#786535}

chrome/browser/extensions/api/extension_action/extension_action_api.cc

@@ -150,8 +150,6 @@ void ExtensionActionAPI::DispatchExtensionActionClicked(
   const char* event_name = NULL;
   switch (extension_action.action_type()) {
     case ActionInfo::TYPE_ACTION:
-      // TODO(https://crbug.com/893373): Add testing for this API (currently
-      // restricted to trunk).
       histogram_value = events::ACTION_ON_CLICKED;
       event_name = "action.onClicked";
       break;