Makes mojo generator generate export for constants

mojo/public/tools/bindings/generators/mojom_cpp_generator.py

 # Copyright 2013 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.
 
 """Generates C++ source files from a mojom.Module."""
 
 import mojom.generate.generator as generator
 import mojom.generate.module as mojom
 import mojom.generate.pack as pack
 from mojom.generate.template_expander import UseJinja
@@ skipping 198 matching lines @@
       return True
   return Check(kind)
 
 
 def GetNativeTypeName(typemapped_kind):
   return _current_typemap[GetFullMojomNameForKind(typemapped_kind)]["typename"]
 
 def GetCppPodType(kind):
   return _kind_to_cpp_type[kind]
 
-def FormatConstantDeclaration(constant, nested=False):
+def FormatConstantDeclaration(constant, nested=False, export_attribute=None):

[yzshen1, 2017/02/28 23:13:09]

Instead of having an export_attribute input, how about defining a global
variable _export_attribute similar to _for_blink and others?

[sky, 2017/02/28 23:45:04]

Done.

   if mojom.IsStringKind(constant.kind):
     if nested:
       return "const char %s[]" % constant.name
-    return "extern const char %s[]" % constant.name
+    return "%sextern const char %s[]" % \
+        ((export_attribute + " ") if export_attribute else "", constant.name)
   return "constexpr %s %s = %s" % (GetCppPodType(constant.kind), constant.name,
                                    ConstantValue(constant))
 
 def GetCppWrapperType(kind, add_same_module_namespaces=False):
   def _AddOptional(type_name):
     pattern = "WTF::Optional<%s>" if _for_blink else "base::Optional<%s>"
     return pattern % type_name
 
   if IsTypemappedKind(kind):
     type_name = GetNativeTypeName(kind)
@@ skipping 495 matching lines @@
       global _use_once_callback
       _use_once_callback = self.use_once_callback
       global _variant
       _variant = self.variant
       suffix = "-%s" % self.variant if self.variant else ""
       self.Write(self.GenerateModuleHeader(),
                  self.MatchMojomFilePath("%s%s.h" % (self.module.name, suffix)))
       self.Write(
           self.GenerateModuleSource(),
           self.MatchMojomFilePath("%s%s.cc" % (self.module.name, suffix)))