Code cleanup for HistogramRemap.
Code cleanup for HistogramRemap.

Avoid call to HistogramAddThresh when there's only one Histogram image.
Change-Id: I43b09e8e2d218c95969567034224777dcce37ab9

appunite/libwebp,Maria1099/webm.libwebp,altogother/webm.libwebp,altogother/webm.libwebp,ya7lelkom/libwebp,abwiz0086/webm.libwebp,imazen/libwebp,ttyangf/libwebp,jacklicn/libwebp,reimaginemedia/webm.libwebp,redengineer/libwebp,ericmckean/webm.libwebp,ya7lelkom/libwebp,jacklicn/libwebp,kleopatra999/webm.libwebp,matanbs/webm.libwebp,zofuthan/libwebp,comoyo/gonzo-libwebp,matanbs/webm.libwebp,comoyo/gonzo-libwebp,zofuthan/libwebp,gshORTON/webm.libwebp,yohunl/libwebp,rxl194/libwebp,PKRoma/libwebp,imazen/libwebp,comoyo/gonzo-libwebp,Aexyn/libwebp,Aexyn/libwebp,kleopatra999/webm.libwebp,Maria1099/webm.libwebp,reimaginemedia/webm.libwebp,rxl194/libwebp,appunite/libwebp,Aexyn/libwebp,Acidburn0zzz/webm.libwebp,zofuthan/libwebp,jacklicn/libwebp,altogother/webm.libwebp,kalli123/webm.libwebp,ttyangf/libwebp,kim42083/webm.libwebp,gshORTON/webm.libwebp,kleopatra999/webm.libwebp,iniwf/webm.libwebp,reimaginemedia/webm.libwebp,kleopatra999/webm.libwebp,kalli123/webm.libwebp,ttyangf/libwebp,redengineer/libwebp,jiehu5114/libwebp,matanbs/webm.libwebp,appunite/libwebp,abwiz0086/webm.libwebp,ericmckean/webm.libwebp,imazen/libwebp,abwiz0086/webm.libwebp,Maria1099/webm.libwebp,iniwf/webm.libwebp,ttyangf/libwebp,iniwf/webm.libwebp,imazen/libwebp,reimaginemedia/webm.libwebp,webmproject/libwebp,kim42083/webm.libwebp,kim42083/webm.libwebp,webmproject/libwebp,ericmckean/webm.libwebp,iniwf/webm.libwebp,Acidburn0zzz/webm.libwebp,zofuthan/libwebp,reimaginemedia/webm.libwebp,yohunl/libwebp,PKRoma/libwebp,rxl194/libwebp,appunite/libwebp,kalli123/webm.libwebp,matanbs/webm.libwebp,iniwf/webm.libwebp,jiehu5114/libwebp,redengineer/libwebp,Suvarna1488/webm.libwebp,jiehu5114/libwebp,yohunl/libwebp,matanbs/webm.libwebp,rxl194/libwebp,ya7lelkom/libwebp,yohunl/libwebp,webmproject/libwebp,comoyo/gonzo-libwebp,comoyo/gonzo-libwebp,Acidburn0zzz/webm.libwebp,Acidburn0zzz/webm.libwebp,Suvarna1488/webm.libwebp,jiehu5114/libwebp,webmproject/libwebp,rxl194/libwebp,ttyangf/libwebp,Maria1099/webm.libwebp,kleopatra999/webm.libwebp,kim42083/webm.libwebp,kalli123/webm.libwebp,ericmckean/webm.libwebp,Aexyn/libwebp,appunite/libwebp,gshORTON/webm.libwebp,Aexyn/libwebp,PKRoma/libwebp,ya7lelkom/libwebp,PKRoma/libwebp,altogother/webm.libwebp,Suvarna1488/webm.libwebp,altogother/webm.libwebp,gshORTON/webm.libwebp,imazen/libwebp,gshORTON/webm.libwebp,ericmckean/webm.libwebp,redengineer/libwebp,PKRoma/libwebp,comoyo/gonzo-libwebp,kalli123/webm.libwebp,Suvarna1488/webm.libwebp,abwiz0086/webm.libwebp,redengineer/libwebp,ya7lelkom/libwebp,abwiz0086/webm.libwebp,jacklicn/libwebp,webmproject/libwebp,zofuthan/libwebp,comoyo/gonzo-libwebp,kim42083/webm.libwebp,PKRoma/libwebp,Maria1099/webm.libwebp,Acidburn0zzz/webm.libwebp,appunite/libwebp,yohunl/libwebp,jacklicn/libwebp,Suvarna1488/webm.libwebp,jiehu5114/libwebp