Bug 1494745 part 4 - Make Loader::AddObserver void since it's infallible. r=bz

mObservers is a nsTObserverArray which is infallible, so no need to
check the return value from AppendElementUnlessExists
(which a later patch in this series will remove).

dom/xml/nsXMLContentSink.cpp

@@ -319,8 +319,8 @@ nsXMLContentSink::DidBuildModel(bool aTerminated)
 
       // We're pretty-printing now.  See whether we should wait up on
       // stylesheet loads
-      if (mDocument->CSSLoader()->HasPendingLoads() &&
-          NS_SUCCEEDED(mDocument->CSSLoader()->AddObserver(this))) {
+      if (mDocument->CSSLoader()->HasPendingLoads()) {
+        mDocument->CSSLoader()->AddObserver(this);
         // wait for those sheets to load
         startLayout = false;
       }

layout/style/Loader.cpp

@@ -2570,15 +2570,11 @@ Loader::HasPendingLoads()
     mDatasToNotifyOn != 0;
 }
 
-nsresult
+void
 Loader::AddObserver(nsICSSLoaderObserver* aObserver)
 {
   MOZ_ASSERT(aObserver, "Must have observer");
-  if (mObservers.AppendElementUnlessExists(aObserver)) {
-    return NS_OK;
-  }
-
-  return NS_ERROR_OUT_OF_MEMORY;
+  mObservers.AppendElementUnlessExists(aObserver);
 }
 
 void

layout/style/Loader.h

@@ -429,7 +429,7 @@ public:
    *
    * aObserver must not be null.
    */
-  nsresult AddObserver(nsICSSLoaderObserver* aObserver);
+  void AddObserver(nsICSSLoaderObserver* aObserver);
 
   /**
    * Remove an observer added via AddObserver.