Call DiscardableMemoryAllocator::SetInstance()
Call DiscardableMemoryAllocator::SetInstance()

Apparently this can cause a crash when when we try to render an emoji at a
different size than the emoji font's native bitmap size. I'm not sure how to
reproduce the issue. I'll ask Hixie when he gets back.

Currently we just use regular memory. In the future, we should actually use
discarable memory.

Fixes #203

qiankunshe/sky_engine,jamesr/flutter_engine,mxia/engine,Hixie/sky_engine,tvolkert/engine,mikejurka/engine,chinmaygarde/flutter_engine,lyceel/engine,devoncarew/engine,mpcomplete/engine,zhangxq5012/sky_engine,TribeMedia/sky_engine,krisgiesing/sky_engine,rmacnak-google/engine,lyceel/engine,zhangxq5012/sky_engine,TribeMedia/sky_engine,mxia/engine,qiankunshe/sky_engine,afandria/sky_engine,afandria/sky_engine,rmacnak-google/engine,mdakin/engine,jason-simmons/flutter_engine,axinging/sky_engine,mdakin/engine,jason-simmons/sky_engine,TribeMedia/sky_engine,TribeMedia/sky_engine,xunmengfeng/engine,aam/engine,zhangxq5012/sky_engine,xunmengfeng/engine,mxia/engine,jamesr/sky_engine,qiankunshe/sky_engine,qiankunshe/sky_engine,afandria/sky_engine,tvolkert/engine,devoncarew/engine,Hixie/sky_engine,mdakin/engine,tvolkert/engine,mikejurka/engine,mxia/engine,rmacnak-google/engine,chinmaygarde/flutter_engine,jason-simmons/sky_engine,abarth/sky_engine,aam/engine,axinging/sky_engine,mikejurka/engine,Hixie/sky_engine,mikejurka/engine,chinmaygarde/flutter_engine,qiankunshe/sky_engine,axinging/sky_engine,Hixie/sky_engine,jason-simmons/flutter_engine,devoncarew/engine,jamesr/flutter_engine,flutter/engine,mdakin/engine,aam/engine,jamesr/sky_engine,afandria/sky_engine,devoncarew/sky_engine,devoncarew/engine,qiankunshe/sky_engine,Hixie/sky_engine,zhangxq5012/sky_engine,mpcomplete/engine,chinmaygarde/sky_engine,krisgiesing/sky_engine,rmacnak-google/engine,Hixie/sky_engine,jason-simmons/flutter_engine,mxia/engine,chinmaygarde/flutter_engine,krisgiesing/sky_engine,chinmaygarde/sky_engine,iansf/sky_engine,cdotstout/sky_engine,cdotstout/sky_engine,jason-simmons/sky_engine,jamesr/flutter_engine,cdotstout/sky_engine,TribeMedia/sky_engine,rmacnak-google/engine,iansf/sky_engine,mdakin/engine,zhangxq5012/sky_engine,aam/engine,flutter/engine,jamesr/sky_engine,chinmaygarde/sky_engine,devoncarew/sky_engine,axinging/sky_engine,mxia/engine,afandria/sky_engine,axinging/sky_engine,xunmengfeng/engine,mpcomplete/engine,lyceel/engine,jamesr/flutter_engine,afandria/sky_engine,jamesr/sky_engine,iansf/sky_engine,mpcomplete/flutter_engine,devoncarew/engine,chinmaygarde/flutter_engine,axinging/sky_engine,devoncarew/sky_engine,abarth/sky_engine,mpcomplete/engine,devoncarew/engine,flutter/engine,xunmengfeng/engine,chinmaygarde/sky_engine,jason-simmons/sky_engine,mdakin/engine,devoncarew/engine,mpcomplete/flutter_engine,iansf/sky_engine,zhangxq5012/sky_engine,tvolkert/engine,jason-simmons/flutter_engine,aam/engine,tvolkert/engine,rmacnak-google/engine,mdakin/engine,chinmaygarde/flutter_engine,abarth/sky_engine,devoncarew/sky_engine,flutter/engine,zhangxq5012/sky_engine,jamesr/flutter_engine,mpcomplete/flutter_engine,flutter/engine,iansf/sky_engine,mikejurka/engine,lyceel/engine,devoncarew/sky_engine,chinmaygarde/sky_engine,jason-simmons/sky_engine,krisgiesing/sky_engine,mxia/engine,jason-simmons/sky_engine,axinging/sky_engine,flutter/engine,mpcomplete/flutter_engine,mikejurka/engine,krisgiesing/sky_engine,flutter/engine,lyceel/engine,xunmengfeng/engine,lyceel/engine,krisgiesing/sky_engine,mpcomplete/engine,iansf/sky_engine,TribeMedia/sky_engine,jamesr/flutter_engine,mikejurka/engine,chinmaygarde/flutter_engine,cdotstout/sky_engine,mikejurka/engine,abarth/sky_engine,tvolkert/engine,abarth/sky_engine,jamesr/sky_engine,tvolkert/engine,chinmaygarde/sky_engine,axinging/sky_engine,zhangxq5012/sky_engine,zhangxq5012/sky_engine,lyceel/engine,cdotstout/sky_engine,Hixie/sky_engine,chinmaygarde/sky_engine,jamesr/flutter_engine,mpcomplete/engine,mxia/engine,mikejurka/engine,abarth/sky_engine,aam/engine,flutter/engine,TribeMedia/sky_engine,afandria/sky_engine,TribeMedia/sky_engine,qiankunshe/sky_engine,cdotstout/sky_engine,afandria/sky_engine,jason-simmons/flutter_engine,jason-simmons/sky_engine,aam/engine,jason-simmons/flutter_engine,mpcomplete/flutter_engine,devoncarew/sky_engine,mdakin/engine,abarth/sky_engine,TribeMedia/sky_engine,iansf/sky_engine,devoncarew/sky_engine,iansf/sky_engine,aam/engine,jamesr/sky_engine,rmacnak-google/engine,jamesr/flutter_engine,jason-simmons/flutter_engine,qiankunshe/sky_engine,mpcomplete/engine,cdotstout/sky_engine,krisgiesing/sky_engine,jamesr/flutter_engine,mpcomplete/engine,xunmengfeng/engine,mpcomplete/flutter_engine,axinging/sky_engine,xunmengfeng/engine,jamesr/sky_engine,Hixie/sky_engine,jason-simmons/flutter_engine