Bug 1293603 (part 2) - Make Run() declarations consistent. r=erahm.

This patch makes most Run() declarations in subclasses of nsIRunnable have the
same form: |NS_IMETHOD Run() override|.

As a result of these changes, I had to add |override| to a couple of other
functions to satisfy clang's -Winconsistent-missing-override warning.

dom/html/HTMLImageElement.cpp

@@ -92,7 +92,7 @@ public:
     mDocument->BlockOnload();
   }
 
-  NS_IMETHOD Run()
+  NS_IMETHOD Run() override
   {
     if (mElement->mPendingImageLoadTask == this) {
       mElement->mPendingImageLoadTask = nullptr;