Extract EventRouter from ExtensionSystem.

This patch removes event_router accessor from ExtensionSystem.
EventRouter is now a browser context keyed service.

BUG=496281

Review URL: https://codereview.chromium.org/1148983004

Cr-Commit-Position: refs/heads/master@{#332745}
diff --git a/chrome/browser/extensions/extension_service.cc b/chrome/browser/extensions/extension_service.cc
index 160ac64..a59eeb9 100644
--- a/chrome/browser/extensions/extension_service.cc
+++ b/chrome/browser/extensions/extension_service.cc
@@ -2410,8 +2410,8 @@
   if (extensions::BackgroundInfo::HasPersistentBackgroundPage(old)) {
     // Delay installation if the extension listens for the onUpdateAvailable
     // event.
-    return system_->event_router()->ExtensionHasEventListener(
-        extension_id, kOnUpdateAvailableEvent);
+    return extensions::EventRouter::Get(profile_)
+        ->ExtensionHasEventListener(extension_id, kOnUpdateAvailableEvent);
   } else {
     // Delay installation if the extension is not idle.
     return !extensions::util::IsExtensionIdle(extension_id, profile_);