Makes mojo generator generate export for constants

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

Index: mojo/public/tools/bindings/generators/mojom_cpp_generator.py
diff --git a/mojo/public/tools/bindings/generators/mojom_cpp_generator.py b/mojo/public/tools/bindings/generators/mojom_cpp_generator.py
index c75013badf88a19cab5a678a1b77174e20c877fc..35b4a640a5fda743aee27624aded0d51526b336e 100644
--- a/mojo/public/tools/bindings/generators/mojom_cpp_generator.py
+++ b/mojo/public/tools/bindings/generators/mojom_cpp_generator.py
@@ -216,11 +216,12 @@ def GetNativeTypeName(typemapped_kind):
 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))