IndexedDB: Correct transaction start scheduling to match spec

Per spec, exclusive transactions should execute in creation order, not
first-use order. This lets the state machine for transactions to be
simplified.

BUG=322238

Review URL: https://codereview.chromium.org/82573006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237258 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237258