I had to put in an ifdef _WIN32 because no other platform implements the call in nsIFontMetrics
I will have to address this ifdef in the next release (Bug 50998), which means I will probably implement
GetAveCharWidth for all platforms that don't have a native call
Bug 44656 r=kmcclusk
git-svn-id: svn://10.0.0.236/trunk@78138 18797224-902f-48f8-a5cc-f745e15eee43