Fix for EXT-6615 (SL webkit no longer reports "Second Life" in the HTTP_USER_AGENT)

The issue here is that initBrowserWindow() was doing the wrong thing if it got called after the set_user_agent message was received.

Made the handler for the set_user_agent message save the string in a member variable (mUserAgent).

Initialize mUserAgent to the default string in the MediaPluginWebKit constructor.

initBrowserWindow() now sets the user agent string from mUserAgent instead of from a literal.