Add comments about what should and should not depend on 'allocator'

base/allocator/allocator.gyp

 # Copyright (c) 2012 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.
 
 {
   'variables': {
     'jemalloc_dir': '../../third_party/jemalloc/chromium',
     'tcmalloc_dir': '../../third_party/tcmalloc/chromium',
     'use_vtable_verify%': 0,
   },
   'targets': [
+    # Only executables and not libraries should depend on the
+    # allocator target; only the application (the final executable)
+    # knows what allocator makes sense.
     {
       'target_name': 'allocator',
       'type': 'static_library',
       # Make sure the allocation library is optimized to
       # the hilt in official builds.
       'variables': {
         'optimize': 'max',
       },
       'include_dirs': [
         '.',
@@ skipping 637 matching lines @@
           'sources': [
             'type_profiler_map_unittests.cc',
             '<(tcmalloc_dir)/src/gperftools/type_profiler_map.h',
             '<(tcmalloc_dir)/src/type_profiler_map.cc',
           ],
         },
       ],
     }],
   ],
 }