Bug 1376638 - Minimize uses of prmem.h. r=glandium.

It's silly to use prmem.h within Firefox code given that in our configuration
its functions are just wrappers for malloc() et al. (Indeed, in some places we
mix PR_Malloc() with free(), or malloc() with PR_Free().)

This patch removes all uses, except for the places where we need to use
PR_Free() to free something allocated by another NSPR function; in those cases
I've added a comment explaining which function did the allocation.

dom/plugins/base/nsNPAPIPluginInstance.cpp

@@ -11,7 +11,6 @@
 #endif
 
 #include "mozilla/Logging.h"
-#include "prmem.h"
 #include "nscore.h"
 #include "prenv.h"
 
@@ -167,7 +166,7 @@ nsNPAPIPluginInstance::~nsNPAPIPluginInstance()
 #endif
 
   if (mMIMEType) {
-    PR_Free((void *)mMIMEType);
+    free(mMIMEType);
     mMIMEType = nullptr;
   }