Allow clusters to start with zero-width characters
Allow clusters to start with zero-width characters

The logic in getRunAdvance() assumed that any zero-width character was
part of the preceding cluster, which is valid most of the time. However,
characters such as ZWNBSP (U+FEFF) renders as a zero width glyph
and is also a grapheme cluster boundary. This patch adds a clause to
handle that case.

Bug: 22121742
Change-Id: Iad79a7d988bded1ef05f0fd7905d20669ea22051

jamesr/flutter_engine,chinmaygarde/sky_engine,krisgiesing/sky_engine,mikejurka/engine,tvolkert/engine,jamesr/sky_engine,mikejurka/engine,jason-simmons/sky_engine,tvolkert/engine,jason-simmons/sky_engine,chinmaygarde/sky_engine,jason-simmons/sky_engine,devoncarew/sky_engine,devoncarew/engine,jason-simmons/sky_engine,jamesr/sky_engine,chinmaygarde/flutter_engine,jason-simmons/flutter_engine,rmacnak-google/engine,mikejurka/engine,devoncarew/sky_engine,jamesr/flutter_engine,rmacnak-google/engine,jason-simmons/flutter_engine,aam/engine,flutter/engine,cdotstout/sky_engine,rmacnak-google/engine,devoncarew/sky_engine,Hixie/sky_engine,flutter/engine,jason-simmons/flutter_engine,Hixie/sky_engine,flutter/engine,cdotstout/sky_engine,mikejurka/engine,Hixie/sky_engine,devoncarew/sky_engine,devoncarew/sky_engine,chinmaygarde/flutter_engine,flutter/engine,aam/engine,jamesr/sky_engine,cdotstout/sky_engine,aam/engine,tvolkert/engine,cdotstout/sky_engine,aam/engine,devoncarew/engine,aam/engine,devoncarew/engine,chinmaygarde/flutter_engine,devoncarew/engine,krisgiesing/sky_engine,jamesr/sky_engine,mikejurka/engine,Hixie/sky_engine,flutter/engine,chinmaygarde/flutter_engine,tvolkert/engine,Hixie/sky_engine,krisgiesing/sky_engine,chinmaygarde/sky_engine,mikejurka/engine,cdotstout/sky_engine,jason-simmons/flutter_engine,jason-simmons/flutter_engine,cdotstout/sky_engine,devoncarew/engine,jamesr/flutter_engine,jamesr/flutter_engine,jason-simmons/flutter_engine,chinmaygarde/flutter_engine,chinmaygarde/sky_engine,aam/engine,mikejurka/engine,jamesr/flutter_engine,krisgiesing/sky_engine,tvolkert/engine,chinmaygarde/sky_engine,devoncarew/engine,krisgiesing/sky_engine,mikejurka/engine,devoncarew/sky_engine,Hixie/sky_engine,jason-simmons/flutter_engine,flutter/engine,krisgiesing/sky_engine,jason-simmons/sky_engine,devoncarew/sky_engine,tvolkert/engine,krisgiesing/sky_engine,jamesr/sky_engine,aam/engine,Hixie/sky_engine,devoncarew/engine,flutter/engine,rmacnak-google/engine,flutter/engine,jason-simmons/sky_engine,jason-simmons/sky_engine,tvolkert/engine,cdotstout/sky_engine,jamesr/flutter_engine,jamesr/flutter_engine,jamesr/sky_engine,rmacnak-google/engine,Hixie/sky_engine,jamesr/sky_engine,rmacnak-google/engine,chinmaygarde/sky_engine,aam/engine,chinmaygarde/flutter_engine,rmacnak-google/engine,chinmaygarde/sky_engine,mikejurka/engine,jamesr/flutter_engine,jamesr/flutter_engine,jason-simmons/flutter_engine,chinmaygarde/flutter_engine