Bug 1411893 - Introduce nsStaticAtom. r=emilio,froydnj.

It's a sub-class of nsAtom, useful for cases where you know you are dealing
exclusively with static atoms. The nice thing about it is that you can use
raw nsStaticAtom pointers instead of RefPtr<>. (In fact, the AddRef/Release
implementations ensure that we'll crash if we use RefPtr<nsStaticAtom>.)

MozReview-Commit-ID: 4Q6QHX5h44V

parser/html/nsHtml5AtomTable.cpp

@@ -50,7 +50,7 @@ nsHtml5AtomTable::GetAtom(const nsAString& aKey)
     return cachedAtom;
   }
 
-  nsAtom* atom = NS_GetStaticAtom(aKey);
+  nsStaticAtom* atom = NS_GetStaticAtom(aKey);
   if (atom) {
     mRecentlyUsedParserAtoms[index] = atom;
     return atom;