Code inspection notes:

* IndexedDBTransaction::Commit() has a special case for !used_ (no requests made), and just commits immediately even if not STARTED. 
* IndexedDBTransaction::Start() bails early if !used_. 
* Watch out for a potential rentrancy problem: IndexedDBTransactionCoordinator::ProcessQueuedTransactions calls IndexedDBTransaction::Start() when iterating over transactions; if that calls Commit() we could end up calling IndexedDBTransactionCoordinator::DidFinishTransaction() and re-enter ProcessQueuedTransactions

One thought: 
* In Commit(), if (!used_ && state_ != STARTED), set commit_pending_ = true then bail
* In Start(), if (!used_ && commit_pending_), PostTask(Commit, this)

This could potentially be optimized by having Start() return a bool and have the coordinator not add the tx to the running transactions, but that's nasty code for a rare case.

PR for WPT: https://github.com/w3c/web-platform-tests/pull/3154

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/d546a9d9162ce273c8b789306ae934b5db9ed6d1

commit d546a9d9162ce273c8b789306ae934b5db9ed6d1
Author: jsbell <jsbell@chromium.org>
Date: Wed Jun 22 20:54:15 2016

IndexedDB: Ensure transactions without requests commit in the same order

In the IDB back-end, a Commit() request that came through for a
transaction that never had any requests was allowed to run the commit
steps without having seen a Start() call from the transaction
coordinator. While the operations would never run out of order, the
commit would happens unexpectedly early from the point of view of
script - i.e. before an earlier transaction with overlapping scope has
completed.

Handle this special case, using the existing commit_pending_ flag
and a new test in Start().

R=palakj@chromium.org,cmumford@chromium.org
BUG= 614530 

Review-Url: https://codereview.chromium.org/2083203002
Cr-Commit-Position: refs/heads/master@{#401395}

[modify] https://crrev.com/d546a9d9162ce273c8b789306ae934b5db9ed6d1/content/browser/indexed_db/indexed_db_transaction.cc
[add] https://crrev.com/d546a9d9162ce273c8b789306ae934b5db9ed6d1/third_party/WebKit/LayoutTests/storage/indexeddb/empty-transaction-order.html