Implement transaction cleanup hook for [HTML]

This defines a hook "cleanup IndexedDB transactions" that HTML
can invoke at the end of tasks, replacing the monkey patch 
"When control is returned to the event loop, the implementation 
must unset the active flag".

index.bs

@@ -966,6 +966,9 @@ if new [=requests=] can be made against the transaction. A
 transaction is said to be <dfn>active</dfn> if its [=transaction/active flag=]
 is set.
 
+A [=/transaction=] optionally has a <dfn>cleanup event loop</dfn>
+which is an [=event loop=].
+
 A [=/transaction=] has a <dfn>request list</dfn> of [=requests=]
 which have been made against the transaction.
 
@@ -1144,6 +1147,26 @@ The following constraints define when a [=/transaction=] can be
 
 </div>
 
+To <dfn export>cleanup Indexed Database transactions</dfn>,
+run the following steps for each [=/transaction=]
+with [=transaction/cleanup event loop=] matching the current
+[=event loop=].
+
+<div class=algorithm>
+    1. Unset the [=/transaction=]'s [=transaction/active flag=].
+
+    2. Clear the [=/transaction=]'s [=transaction/cleanup event loop=].
+</div>
+
+<aside class=note>
+  This behavior is invoked by [[HTML]]. It ensures that
+  [=/transactions=] created by a script call
+  to{{IDBDatabase/transaction()}} are deactivated once the task that
+  invoked the script has completed. The steps are run at most once for
+  each [=/transaction=].
+</aside>
+
+
 <!-- ============================================================ -->
 <h4 id=upgrade-transaction-construct>Upgrade Transactions</h4>
 <!-- ============================================================ -->
@@ -2550,8 +2573,8 @@ The <dfn method for=IDBDatabase>transaction(|storeNames|,
     |connection|, |mode| and the set of [=/object stores=] named in
     |scope|.
 
-8. When control is returned to the [=event loop=], the implementation must
-    unset the [=transaction/active flag=].
+8. Set |transaction|'s [=transaction/cleanup event loop=] to the
+    current [=event loop=].
 
 9. Return an {{IDBTransaction}} object representing |transaction|.
 
@@ -5457,11 +5480,13 @@ for the [=database=], and a |request|.
     1. Set |request|'s [=request/result=] to |connection|.
     2. Set |request|'s [=request/transaction=] to |transaction|.
     3. Set the [=request/done flag=] on the [=request=].
-    4. Let |didThrow| be the result of running the steps to
+    4. Set |transaction|'s [=transaction/active flag=].
+    5. Let |didThrow| be the result of running the steps to
         [=fire a version change event=] named
         <code>[=upgradeneeded=]</code> at |request| with |old
         version| and |version|.
-    5. If |didThrow| is set, run the steps to [=abort a
+    6. Unset |transaction|'s [=transaction/active flag=].
+    7. If |didThrow| is set, run the steps to [=abort a
         transaction=] with the |error| property set to a newly
         <a for=exception>created</a> "{{AbortError}}" {{DOMException}}.