bug 955957 - pt 0 - add a simple unit test for gfxSkipChars functionality. r=roc
bug 955957 - pt 1 - reimplement gfxSkipChars and gfxSkipCharsIterator to perform better with huge text runs. r=roc
bug 955957 - pt 1.1 - update gfxSkipChars test for revised API (no gfxSkipCharsBuilder any longer). r=roc