Auto merge of #12906 - servo:ffiless, r=emilio
Auto merge of #12906 - servo:ffiless, r=emilio

Remove some use of FFI in gecko_string_cache

Now that nsIAtom’s mLength and mString can both be accessed directly on the struct, fewer operations need to call into C++ code.

These functions are not used anymore and can be removed from the Gecko side:

* `Gecko_GetAtomAsUTF16`
* `Gecko_AtomEqualsUTF8IgnoreCase`
* `Gecko_AtomEqualsUTF8` (looks like this one was already unused before this PR)

r? @emilio

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dsandeephegde/servo,thiagopnts/servo,larsbergstrom/servo,emilio/servo,pyfisch/servo,avadacatavra/servo,rnestler/servo,cbrewster/servo,canaltinova/servo,anthgur/servo,saneyuki/servo,peterjoel/servo,fiji-flo/servo,CJ8664/servo,mbrubeck/servo,mattnenterprise/servo,upsuper/servo,upsuper/servo,KiChjang/servo,SimonSapin/servo,ConnorGBrewster/servo,szeged/servo,splav/servo,pyfisch/servo,thiagopnts/servo,jimberlage/servo,pyfisch/servo,jimberlage/servo,dati91/servo,mbrubeck/servo,larsbergstrom/servo,larsbergstrom/servo,thiagopnts/servo,KiChjang/servo,splav/servo,KiChjang/servo,szeged/servo,paulrouget/servo,ConnorGBrewster/servo,cbrewster/servo,peterjoel/servo,mbrubeck/servo,szeged/servo,cbrewster/servo,sadmansk/servo,KiChjang/servo,mattnenterprise/servo,eddyb/servo,jimberlage/servo,pyfisch/servo,upsuper/servo,saneyuki/servo,notriddle/servo,splav/servo,thiagopnts/servo,CJ8664/servo,notriddle/servo,szeged/servo,emilio/servo,saneyuki/servo,paulrouget/servo,SimonSapin/servo,ConnorGBrewster/servo,mattnenterprise/servo,SimonSapin/servo,emilio/servo,upsuper/servo,nnethercote/servo,paulrouget/servo,mattnenterprise/servo,canaltinova/servo,avadacatavra/servo,dati91/servo,canaltinova/servo,CJ8664/servo,larsbergstrom/servo,canaltinova/servo,ConnorGBrewster/servo,nnethercote/servo,CJ8664/servo,KiChjang/servo,ConnorGBrewster/servo,mattnenterprise/servo,saneyuki/servo,DominoTree/servo,DominoTree/servo,avadacatavra/servo,dati91/servo,anthgur/servo,eddyb/servo,emilio/servo,avadacatavra/servo,dsandeephegde/servo,dati91/servo,saneyuki/servo,SimonSapin/servo,larsbergstrom/servo,upsuper/servo,szeged/servo,eddyb/servo,fiji-flo/servo,ConnorGBrewster/servo,splav/servo,notriddle/servo,peterjoel/servo,emilio/servo,saneyuki/servo,thiagopnts/servo,paulrouget/servo,saneyuki/servo,dati91/servo,SimonSapin/servo,fiji-flo/servo,mbrubeck/servo,mattnenterprise/servo,larsbergstrom/servo,cbrewster/servo,mbrubeck/servo,jimberlage/servo,pyfisch/servo,avadacatavra/servo,cbrewster/servo,larsbergstrom/servo,thiagopnts/servo,pyfisch/servo,saneyuki/servo,dsandeephegde/servo,larsbergstrom/servo,notriddle/servo,nnethercote/servo,nnethercote/servo,avadacatavra/servo,DominoTree/servo,szeged/servo,mbrubeck/servo,mbrubeck/servo,eddyb/servo,dsandeephegde/servo,KiChjang/servo,canaltinova/servo,jimberlage/servo,eddyb/servo,thiagopnts/servo,DominoTree/servo,sadmansk/servo,canaltinova/servo,paulrouget/servo,rnestler/servo,rnestler/servo,pyfisch/servo,paulrouget/servo,rnestler/servo,fiji-flo/servo,pyfisch/servo,peterjoel/servo,sadmansk/servo,KiChjang/servo,anthgur/servo,splav/servo,sadmansk/servo,mattnenterprise/servo,emilio/servo,szeged/servo,dsandeephegde/servo,splav/servo,dsandeephegde/servo,anthgur/servo,CJ8664/servo,notriddle/servo,emilio/servo,ConnorGBrewster/servo,splav/servo,paulrouget/servo,CJ8664/servo,peterjoel/servo,CJ8664/servo,sadmansk/servo,DominoTree/servo,notriddle/servo,nnethercote/servo,nnethercote/servo,paulrouget/servo,SimonSapin/servo,SimonSapin/servo,cbrewster/servo,notriddle/servo,larsbergstrom/servo,nnethercote/servo,mattnenterprise/servo,eddyb/servo,rnestler/servo,SimonSapin/servo,fiji-flo/servo,nnethercote/servo,jimberlage/servo,dsandeephegde/servo,saneyuki/servo,fiji-flo/servo,larsbergstrom/servo,sadmansk/servo,cbrewster/servo,szeged/servo,DominoTree/servo,nnethercote/servo,avadacatavra/servo,dati91/servo,upsuper/servo,KiChjang/servo,DominoTree/servo,notriddle/servo,jimberlage/servo,emilio/servo,CJ8664/servo,KiChjang/servo,sadmansk/servo,KiChjang/servo,emilio/servo,peterjoel/servo,eddyb/servo,splav/servo,peterjoel/servo,pyfisch/servo,eddyb/servo,szeged/servo,anthgur/servo,szeged/servo,thiagopnts/servo,dsandeephegde/servo,cbrewster/servo,upsuper/servo,ConnorGBrewster/servo,nnethercote/servo,paulrouget/servo,fiji-flo/servo,anthgur/servo,DominoTree/servo,peterjoel/servo,upsuper/servo,splav/servo,notriddle/servo,saneyuki/servo,sadmansk/servo,canaltinova/servo,emilio/servo,DominoTree/servo,anthgur/servo,rnestler/servo,dati91/servo,pyfisch/servo,jimberlage/servo,peterjoel/servo,paulrouget/servo,jimberlage/servo,anthgur/servo,rnestler/servo,canaltinova/servo,avadacatavra/servo,mbrubeck/servo,notriddle/servo,peterjoel/servo,splav/servo,dati91/servo,fiji-flo/servo,DominoTree/servo,rnestler/servo