Baseline metrics now w/correct sign; Added check before letting native impl through
The signs of emHeightAscent, emHeightDescent, hangingBaseline, alphabeticBaseline, and ideographicBaseline now comply with the standard.
This also breaks compatibility with Chrome Canary's buggy implementation. See comments on https://code.google.com/p/chromium/issues/detail?id=277215
In light of there being no correct native implementations, the polyfill now checks for the known bugs in Chrome at load time and overrides the native measureText if they seem to be present.