Give "exit fullscreen" a fully flag to replace "fully exit fullscreen"

The previous definition of fully exit fullscreen could unfullscreen
elements in top layer synchronously. If those elements were iframes,
the contentDocument would get handled in a following animation frame
task, leaving everything in an odd state in the interim.

Exposing the concept of fully exiting to scripts is a separate issue:
#70

fullscreen.bs

@@ -85,18 +85,6 @@ its <a>node document</a>'s <a>top layer</a>.
 
 <hr>
 
-<p>To <dfn>fully exit fullscreen</dfn> a <a for=/>document</a> <var>document</var>, run these steps:
-
-<ol>
- <li><p>If <var>document</var>'s <a>fullscreen element</a> is null, terminate these steps.
-
- <li><p><a lt="Unfullscreen an element">Unfullscreen elements</a> whose <a>fullscreen flag</a> is
- set, within <var>document</var>'s <a>top layer</a>, except for <var>document</var>'s
- <a>fullscreen element</a>.
-
- <li><p><a>Exit fullscreen</a> <var>document</var>.
-</ol>
-
 <p>Whenever the <a>removing steps</a> run with a <var>removedNode</var>, run these steps:
 
 <ol>
@@ -109,15 +97,15 @@ its <a>node document</a>'s <a>top layer</a>.
 
   <ol>
    <li><p>If <var>node</var> is its <a>node document</a>'s <a>fullscreen element</a>,
-   <a>exit fullscreen</a> that <a for=/>document</a>.
+   <a>exit fullscreen</a> that <a for=/>document</a> with <var>fullyFlag</var> unset.
 
    <li><p>Otherwise, <a lt="unfullscreen an element">unfullscreen <var>node</var></a> within its
    <a>node document</a>.
   </ol>
 </ol>
 
 <p>Whenever the <a>unloading document cleanup steps</a> run with a <var>document</var>,
-<a>fully exit fullscreen</a> <var>document</var>.
+<a>exit fullscreen</a> <var>document</var> with <var>fullyFlag</var> set.
 
 <hr>
 
@@ -343,7 +331,8 @@ could be an open <{dialog}> element.
  in which case that document will still remain in fullscreen.
 </ol>
 
-<p>To <dfn>exit fullscreen</dfn> a <a for=/>document</a> <var>doc</var>, run these steps:
+<p>To <dfn>exit fullscreen</dfn> a <a for=/>document</a> <var>doc</var> with
+<var>fullyFlag</var>, run these steps:
 
 <ol>
  <li><p>Let <var>promise</var> be a new promise.
@@ -385,8 +374,8 @@ could be an open <{dialog}> element.
 
    <li><p>If <var>resize</var> is true and <var>topLevelDoc</var> is either not in
    <var>exitDocs</var>, or not a <a>simple fullscreen document</a>,
-   <a>fully exit fullscreen</a> <var>topLevelDoc</var>, reject <var>promise</var> with a
-   <code>TypeError</code> exception, and terminate these steps.
+   <a>exit fullscreen</a> <var>topLevelDoc</var> with <var>fullyFlag</var> set, reject
+   <var>promise</var> with a <code>TypeError</code> exception, and terminate these steps.
    <!-- cross-process -->
 
    <li><p>Let <var>descendantDocs</var> be an <a>ordered set</a> consisting of <var>doc</var>'s
@@ -398,8 +387,16 @@ could be an open <{dialog}> element.
    <a lt="unfullscreen a document">unfullscreen <var>descendantDoc</var></a>.
    <!-- cross-process -->
 
-   <li><p><a>For each</a> <var>exitDoc</var> in <var>exitDocs</var>,
-   <a lt="unfullscreen an element">unfullscreen</a> <var>exitDoc</var>'s <a>fullscreen element</a>.
+   <li>
+    <p><a>For each</a> <var>exitDoc</var> in <var>exitDocs</var>:
+
+    <ol>
+     <li><p>If <var>exitDoc</var> is <var>doc</var> and <var>fullyFlag</var> is set, then
+     <a lt="unfullscreen a document">unfullscreen <var>doc</var></a>.
+
+     <li><p>Otherwise, <a lt="unfullscreen an element">unfullscreen</a> <var>exitDoc</var>'s
+     <a>fullscreen element</a>.
+    </ol>
 
    <li><p><a>For each</a> <var>descendantDoc</var> in <var>descendantDocs</var>,
    <a>fire an event</a> named <code>fullscreenchange</code> on <var>descendantDoc</var>.
@@ -416,7 +413,8 @@ could be an open <{dialog}> element.
 </ol>
 
 <p>The <dfn method for=Document><code>exitFullscreen()</code></dfn> method, when invoked, must
-return the result of running <a>exit fullscreen</a> on the <a>context object</a>.
+return the result of running <a>exit fullscreen</a> on the <a>context object</a> with
+<var>fullyFlag</var> unset.
 
 <hr>
 
@@ -445,8 +443,8 @@ return the result of running <a>exit fullscreen</a> on the <a>context object</a>
 {{Element/requestFullscreen()}} and {{Document/exitFullscreen()}}.
 
 <p>If the end user instructs the user agent to end a fullscreen session initiated via
-{{Element/requestFullscreen()}}, <a>fully exit fullscreen</a> the
-<a>top-level browsing context</a>'s <a>active document</a>.
+{{Element/requestFullscreen()}}, <a>exit fullscreen</a> the
+<a>top-level browsing context</a>'s <a>active document</a> with <var>fullyFlag</var> set.