changed nsRenderingContext API, and let GetMetricsFor take lang group as a required argument. Changed all such calls. Implement a convenient function to simplifed the code. r=rbs, sr=waterson, a=adt, rjesup git-svn-id: svn://10.0.0.236/branches/MOZILLA_1_0_BRANCH@122452 18797224-902f-48f8-a5cc-f745e15eee43