Xqyin color messages

Modify HtmlKeywords::WritePre function to take a new argument. The new argument adds a style attribute to the pre-tag. Here use the empty string to indicate that style attribute won't be used. 

To add a style attribute is to color messages in message_history page. That part of codes can be viewed in net/instaweb/system/system_server_context.cc.

The corresponding system test is added. A warning message is injected before the test to make sure that there is at least one warning message in the message_history page.