linux: split libxpcom_hack into its own file

It's also empty; this works around a scons warning.

Review URL: http://codereview.chromium.org/202029

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@25785 0039d316-1c4b-4281-b951-d872f2087c98

webkit/tools/libxul_hack/libxpcom_hack.cc

+// Copyright (c) 2009 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// See libxul_hack.cc.  This additional file works around scons not liking
+// multiple targets from the same source.

webkit/webkit.gyp

@@ -1625,7 +1625,7 @@
           'product_dir': '<(PRODUCT_DIR)',
           'type': 'loadable_module',
           'sources': [
-            'tools/libxul_hack/libxul_hack.cc'
+            'tools/libxul_hack/libxpcom_hack.cc'
           ],
         },
       ], # targets