PPAPI: Add an API for setting a custom user agent for URL requests.

ppapi/shared_impl/ppb_url_request_info_shared.h

Index: ppapi/shared_impl/ppb_url_request_info_shared.h
diff --git a/ppapi/shared_impl/ppb_url_request_info_shared.h b/ppapi/shared_impl/ppb_url_request_info_shared.h
index dbad103899b20a4fae416b3baeb35d8c184adda7..a233708f6586fcdabc0ddabb11ddbf9fa3cd256a 100644
--- a/ppapi/shared_impl/ppb_url_request_info_shared.h
+++ b/ppapi/shared_impl/ppb_url_request_info_shared.h
@@ -73,9 +73,11 @@ struct PPAPI_SHARED_EXPORT PPB_URLRequestInfo_Data {
   bool allow_credentials;
 
   // Similar to the custom referrer (above), but for custom content transfer
-  // encoding.
+  // encoding and custom user agent, respectively.
   bool has_custom_content_transfer_encoding;
   std::string custom_content_transfer_encoding;
+  bool has_custom_user_agent;
+  std::string custom_user_agent;
 
   int32_t prefetch_buffer_upper_threshold;
   int32_t prefetch_buffer_lower_threshold;