Expose aggressiveTileRetentionEnabled in WKPreferences SPI to match C SPI
<rdar://problem/38675155>
Created attachment 336143 [details] Patch
Comment on attachment 336143 [details] Patch Clearing flags on attachment: 336143 Committed r229773: <https://trac.webkit.org/changeset/229773>
All reviewed patches have been landed. Closing bug.