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.

netwerk/cache/nsDiskCacheDeviceSQL.cpp

@@ -258,7 +258,7 @@ public:
   {
   }
 
-  NS_IMETHOD Run()
+  NS_IMETHOD Run() override
   {
     if (mDevice->IsActiveCache(mGroup, mClientID))
     {
@@ -875,7 +875,7 @@ public:
     mDB = aDB;
   }
 
-  NS_IMETHOD Run()
+  NS_IMETHOD Run() override
   {
     mDB->Close();
     return NS_OK;