IndexedDB: Implement explicit queue for IDBOpenDBRequests

Align Chrome's processing of IDB open() and deleteDatabase() requests
with other browsers: such requests should be processed in FIFO order.
Previously, Chrome let delete requests jump ahead of blocked opens, as
hinted at in the spec but not supported by other browsers.

R=cmumford@chromium.org
BUG=479388

Review-Url: https://codereview.chromium.org/2148253003
Cr-Commit-Position: refs/heads/master@{#405880}
8 files changed