Use _GLIBCXX_DEBUG in standalone Debug builds on Linux.

build/standalone.gypi

 # Copyright 2014 PDFium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # Definitions to be used when building stand-alone PDFium binaries.
 
 {
   'variables': {
     'component%': 'static_library',
     'clang%': 0,
@@ skipping 81 matching lines @@
               }],
             ],
           },
           'VCLinkerTool': {
             'LinkIncremental': '2',
           },
         },
         'xcode_settings': {
           'GCC_OPTIMIZATION_LEVEL': '0',  # -O0
         },
+        'conditions': [
+          ['OS=="linux"', {
+            # Enable libstdc++ debugging to help catch problems early.
+            'defines': ['_GLIBCXX_DEBUG=1',],
+          }],
+        ],
       },
       'Release': {
         'cflags': [
           '-fno-strict-aliasing',
         ],
         'xcode_settings': {
           'GCC_OPTIMIZATION_LEVEL': '3',  # -O3
           'GCC_STRICT_ALIASING': 'NO',
         },
         'msvs_settings': {
@@ skipping 327 matching lines @@
       ],
     }],  # OS=="linux" or OS=="mac"
     ["use_goma==1", {
       'make_global_settings': [
         ['CC_wrapper', '<(gomadir)/gomacc'],
         ['CXX_wrapper', '<(gomadir)/gomacc'],
       ],
     }],  # use_goma==1
   ],
 }