[Cronet] Add support for constants in cronet idl.

This adds support needed by crrev.com/c/1374158.

Bug: 879208
Change-Id: I80894a166cf4233087b81a455cfc22a106f724e6
Reviewed-on: https://chromium-review.googlesource.com/c/1382694Reviewed-by: Paul Jensen <pauljensen@chromium.org>
Reviewed-by: Misha Efimov <mef@chromium.org>
Commit-Queue: Caleb Raitto <caraitto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619429}

components/cronet/tools/generators/c_templates/module_c.h.tmpl

@@ -62,6 +62,11 @@ typedef enum {{enum_name}} {
 
 {%   endfor %}
 
+// Declare constants
+{%- for constant in module.constants %}
+{{constant|format_constant_declaration}};
+{%- endfor %}
+
 {#--- Interface Stubs -#}
 {%  for interface in interfaces %}
 {%- set interface_name = interface|get_name_for_kind %}

components/cronet/tools/generators/cronet_c_generator.py

@@ -537,8 +537,8 @@ class Generator(generator.Generator):
       return "%sextern const char %s[]" % \
           ((self.export_attribute + " ") if self.export_attribute else "",
            constant.name)
-    return "constexpr %s %s = %s" % (
-        GetCppPodType(constant.kind), constant.name,
+    return "const %s %s_%s = %s" % (
+        GetCppPodType(constant.kind), self.module.namespace, constant.name,
         self._ConstantValue(constant))
 
   def _GetCppWrapperType(self, kind, add_same_module_namespaces=False):