Non-string constants in structs and interfaces need a definition too.

Without this ODR-using these constants will cause linking errors.
Namespace scoped constants are fine without a definition, since those
have internal linkage and thus every user gets their own copy of the
constant anyway.

Bug: 
Change-Id: I361fc705500d67f010498208622022e81a20b2eb
Reviewed-on: https://chromium-review.googlesource.com/521767Reviewed-by: Yuzhu Shen <yzshen@chromium.org>
Commit-Queue: Marijn Kruisselbrink <mek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#476534}

mojo/public/tools/bindings/generators/cpp_templates/interface_definition.tmpl

@@ -50,6 +50,8 @@ const char {{class_name}}::Name_[] = "{{namespace_as_string}}::{{class_name}}";
 {%-  for constant in interface.constants %}
 {%-   if constant.kind|is_string_kind %}
 const char {{interface.name}}::{{constant.name}}[] = {{constant|constant_value}};
+{%-   else %}
+constexpr {{constant.kind|cpp_pod_type}} {{interface.name}}::{{constant.name}};
 {%-   endif %}
 {%- endfor %}
 

mojo/public/tools/bindings/generators/cpp_templates/module.cc.tmpl

@@ -59,6 +59,8 @@ const char {{constant.name}}[] = {{constant|constant_value}};
 {%-   for constant in struct.constants %}
 {%-     if constant.kind|is_string_kind %}
 const char {{struct.name}}::{{constant.name}}[] = {{constant|constant_value}};
+{%-     else %}
+constexpr {{constant.kind|cpp_pod_type}} {{struct.name}}::{{constant.name}};
 {%-     endif %}
 {%-   endfor %}
 {%- endfor %}