Bug 1212027 - part 6 - add an IsEmpty method to nsTHashtable; r=erahm

It's a little more convenient than checking Count(), and also gives
nsTHashtable the same interface as nsTArray (for this operation, at
least), which seems worthwhile.

xpcom/glue/nsTHashtable.h

@@ -119,6 +119,11 @@ public:
    */
   uint32_t Count() const { return mTable.EntryCount(); }
 
+  /**
+   * Return true if the hashtable is empty.
+   */
+  bool IsEmpty() const { return Count() == 0; }
+
   /**
    * Get the entry associated with a key.
    * @param     aKey the key to retrieve