Bug 1293117 (part 4) - Change many NS_IMETHODIMP occurrences to NS_IMETHOD. r=froydnj.

This patch makes the following changes on many in-class methods.

- NS_IMETHODIMP F() override;      --> NS_IMETHOD F() override;
- NS_IMETHODIMP F() override {...} --> NS_IMETHOD F() override {...}
- NS_IMETHODIMP F() final;         --> NS_IMETHOD F() final;
- NS_IMETHODIMP F() final {...}    --> NS_IMETHOD F() final {...}

Using NS_IMETHOD is the preferred way of marking in-class virtual methods.
Although these transformations add an explicit |virtual|, they are safe --
there's an implicit |virtual| anyway because |override| and |final| only work
with virtual methods.

parser/html/nsHtml5Module.cpp

@@ -92,7 +92,7 @@ class nsHtml5ParserThreadTerminator final : public nsIObserver
     explicit nsHtml5ParserThreadTerminator(nsIThread* aThread)
       : mThread(aThread)
     {}
-    NS_IMETHODIMP Observe(nsISupports *, const char *topic, const char16_t *) override
+    NS_IMETHOD Observe(nsISupports *, const char *topic, const char16_t *) override
     {
       NS_ASSERTION(!strcmp(topic, "xpcom-shutdown-threads"), 
                    "Unexpected topic");