Bug 1172782 - Change how nsTHashtable::Clear() works. r=froydnj.

This change reimplements nsTHashtable::Clear() using PLDHashable::Clear(). This
changes its semantics slightly -- the old version would clear the table but
leave its capacity unchanged. The new version will adjust the capacity
to the default, though the entry storage will only be re-allocated when the
first new element is added.

xpcom/glue/nsTHashtable.cpp

@@ -1,16 +0,0 @@
-/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
-/* vim: set ts=8 sts=2 et sw=2 tw=80: */
-/* This Source Code Form is subject to the terms of the Mozilla Public
- * License, v. 2.0. If a copy of the MPL was not distributed with this
- * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
-
-#include "nsTHashtable.h"
-
-PLDHashOperator
-PL_DHashStubEnumRemove(PLDHashTable*    aTable,
-                       PLDHashEntryHdr* aEntry,
-                       uint32_t         aOrdinal,
-                       void*            aUserarg)
-{
-  return PL_DHASH_REMOVE;
-}

xpcom/glue/nsTHashtable.h

@@ -19,13 +19,6 @@
 
 #include <new>
 
-// helper function for nsTHashtable::Clear()
-PLDHashOperator PL_DHashStubEnumRemove(PLDHashTable* aTable,
-                                       PLDHashEntryHdr* aEntry,
-                                       uint32_t aOrdinal,
-                                       void* aUserArg);
-
-
 /**
  * a base class for templated hashtables.
  *
@@ -211,11 +204,13 @@ public:
   }
 
   /**
-   * remove all entries, return hashtable to "pristine" state ;)
+   * Remove all entries, return hashtable to "pristine" state. It's
+   * conceptually the same as calling the destructor and then re-calling the
+   * constructor.
    */
   void Clear()
   {
-    PL_DHashTableEnumerate(&mTable, PL_DHashStubEnumRemove, nullptr);
+    mTable.Clear();
   }
 
   /**

xpcom/glue/objs.mozbuild

@@ -25,7 +25,6 @@ xpcom_glue_src_lcppsrcs = [
     'nsMemory.cpp',
     'nsQuickSort.cpp',
     'nsTArray.cpp',
-    'nsTHashtable.cpp',
     'nsThreadUtils.cpp',
     'nsTObserverArray.cpp',
     'nsVersionComparator.cpp',

xpcom/libxpcomrt/moz.build

@@ -77,7 +77,6 @@ xpcom_glue_src = [
     'nsProxyRelease.cpp',
     'nsQuickSort.cpp',
     'nsTArray.cpp',
-    'nsTHashtable.cpp',
     'nsTObserverArray.cpp',
     'nsThreadUtils.cpp',
     'nsWeakReference.cpp',