Plugin Power Saver: Replace all "plug-in" with "plugin" for consistency.

Passed chrome-ui-review: https://groups.google.com/a/google.com/d/msg/chrome-ui-review/FwuQ2TWc5n8/H3PSuV4L_cAJ

BUG=467165

TBR=tsepez
(comment changes only in render_messages)

NOPRESUBMIT=true
(legacy LOG messages)

Review URL: https://codereview.chromium.org/1007443004

Cr-Commit-Position: refs/heads/master@{#320782}
diff --git a/extensions/browser/process_manager.h b/extensions/browser/process_manager.h
index 16691b4..d897d29 100644
--- a/extensions/browser/process_manager.h
+++ b/extensions/browser/process_manager.h
@@ -120,7 +120,7 @@
   // called regularly.
   void KeepaliveImpulse(const Extension* extension);
 
-  // Triggers a keepalive impulse for a plug-in (e.g NaCl).
+  // Triggers a keepalive impulse for a plugin (e.g NaCl).
   static void OnKeepaliveFromPlugin(int render_process_id,
                                     int render_frame_id,
                                     const std::string& extension_id);