With a further simplification of threading assumptions, we can get rid of atomic integer access as well as the backing store mutex which was otherwise unused. Patch below.
Created attachment 187938 [details] Patch
Created attachment 187944 [details] Patch Apparently we're not RIM anymore, so referencing a RIM PR in the commit message is probably suboptimal. It's now Internal PR. No changes to the patch except for that reference.
Comment on attachment 187944 [details] Patch LGTM.
Comment on attachment 187944 [details] Patch Clearing flags on attachment: 187944 Committed r142682: <http://trac.webkit.org/changeset/142682>
All reviewed patches have been landed. Closing bug.