Introduce a gyp variable to control whether or not slow dchecks are on

Makefile

 # Copyright 2012 the V8 project authors. All rights reserved.
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 #       notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
 #       copyright notice, this list of conditions and the following
 #       disclaimer in the documentation and/or other materials provided
@@ skipping 72 matching lines @@
 ifeq ($(snapshot), external)
   GYPFLAGS += -Dv8_use_external_startup_data=1
 endif
 # extrachecks=on/off
 ifeq ($(extrachecks), on)
   GYPFLAGS += -Ddcheck_always_on=1 -Dv8_enable_handle_zapping=1
 endif
 ifeq ($(extrachecks), off)
   GYPFLAGS += -Ddcheck_always_on=0 -Dv8_enable_handle_zapping=0
 endif
+# slowdchecks=on/off
+ifeq ($(slowdchecks), on)
+  GYPFLAGS += -Dv8_enable_slow_dchecks=1
+endif
+ifeq ($(slowdchecks), off)
+  GYPFLAGS += -Dv8_enable_slow_dchecks=0
+endif
 # gdbjit=on/off
 ifeq ($(gdbjit), on)
   GYPFLAGS += -Dv8_enable_gdbjit=1
 endif
 ifeq ($(gdbjit), off)
   GYPFLAGS += -Dv8_enable_gdbjit=0
 endif
 # vtunejit=on
 ifeq ($(vtunejit), on)
   GYPFLAGS += -Dv8_enable_vtunejit=1
@@ skipping 396 matching lines @@
               third_party/icu --revision 277999 ; \
         fi
         svn checkout --force https://googletest.googlecode.com/svn/trunk \
             testing/gtest --revision 692
         svn checkout --force https://googlemock.googlecode.com/svn/trunk \
             testing/gmock --revision 485
 
 dependencies: builddeps
         # The spec is a copy of the hooks in v8's DEPS file.
         gclient sync -r fb782d4369d5ae04f17a2fceef7de5a63e50f07b --spec="solutions = [{u'managed': False, u'name': u'buildtools', u'url': u'https://chromium.googlesource.com/chromium/buildtools.git', u'custom_deps': {}, u'custom_hooks': [{u'name': u'clang_format_win',u'pattern': u'.',u'action': [u'download_from_google_storage',u'--no_resume',u'--platform=win32',u'--no_auth',u'--bucket',u'chromium-clang-format',u'-s',u'buildtools/win/clang-format.exe.sha1']},{u'name': u'clang_format_mac',u'pattern': u'.',u'action': [u'download_from_google_storage',u'--no_resume',u'--platform=darwin',u'--no_auth',u'--bucket',u'chromium-clang-format',u'-s',u'buildtools/mac/clang-format.sha1']},{u'name': u'clang_format_linux',u'pattern': u'.',u'action': [u'download_from_google_storage',u'--no_resume',u'--platform=linux*',u'--no_auth',u'--bucket',u'chromium-clang-format',u'-s',u'buildtools/linux64/clang-format.sha1']}],u'deps_file': u'.DEPS.git', u'safesync_url': u''}]"