go.mobile/sprite: add AppendChild and RemoveChild from go.net/html
go.mobile/sprite: add AppendChild and RemoveChild from go.net/html

LGTM=nigeltao
R=nigeltao
CC=golang-codereviews
https://golang.org/cl/153680046

ppamorim/mobile,steeve/mobile,goldcaddy77/mobile,ichu501/mobile,ppamorim/mobile,samanalysis/mobile,ppamorim/mobile,golang/mobile,DanielWorld/mobile,freeman0211/mobile,tempbottle/mobile,golang/mobile,MaxCy330/mobile,ichu501/mobile,0x90sled/mobile,cainiaocome/mobile,0x90sled/mobile,cainiaocome/mobile,cainiaocome/mobile,ppamorim/mobile,freeman0211/mobile,freeman0211/mobile,tempbottle/mobile,zach-klippenstein/mobile,lonphy23t/mobile,lonphy23t/mobile,goldcaddy77/mobile,goldcaddy77/mobile,zach-klippenstein/mobile,samanalysis/mobile,MaxCy330/mobile,steeve/mobile,0x90sled/mobile,swisspol/mobile,tempbottle/mobile,freeman0211/mobile,qskycolor/mobile,qskycolor/mobile,zach-klippenstein/mobile,grategames/mobile,tempbottle/mobile,b54898533/mobile,b54898533/mobile,golang/mobile,qskycolor/mobile,b54898533/mobile,0x90sled/mobile,qskycolor/mobile,steeve/mobile,DanielWorld/mobile,MaxCy330/mobile,golang/mobile,lonphy23t/mobile,zach-klippenstein/mobile,xing/gomobile,b54898533/mobile,tempbottle/mobile,MaxCy330/mobile,grategames/mobile,grategames/mobile,swisspol/mobile,ppamorim/mobile,cainiaocome/mobile,swisspol/mobile,goldcaddy77/mobile,goldcaddy77/mobile,MaxCy330/mobile,samanalysis/mobile,samanalysis/mobile,lonphy23t/mobile,lonphy23t/mobile,ichu501/mobile,xing/gomobile,0x90sled/mobile,samanalysis/mobile,DanielWorld/mobile,b54898533/mobile,DanielWorld/mobile,xing/gomobile,xing/gomobile,swisspol/mobile,cainiaocome/mobile,ichu501/mobile,xing/gomobile,ichu501/mobile,swisspol/mobile,steeve/mobile,grategames/mobile