Bug 932162 - dispatch IndexedDB FileInfo releases to the main thread. r=khuey

Try runs:
Green run prior to b2g mochitest fix and assertion changes:
https://tbpl.mozilla.org/?tree=Try&rev=b071f8ef9617
green runs with assertion changes, not b2g disabling:
https://tbpl.mozilla.org/?tree=Try&rev=b071f8ef9617
green b2g run:
https://tbpl.mozilla.org/?tree=Try&rev=67510897d368