Add a 'sessionId' field to GWT lightweight metrics events, which is set to the value of
the user-supplied global variable __gwtStatsSessionId at module startup.  The field should
not be considered a commited part of the API at this time.

Review by: bobv



git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@6441 8db76d5a-ed1c-0410-87a9-c151d255dfc7