Change dir=auto to fall back to ltr rather than inheriting

This restores the old behavior that was part of the spec from November
2010 to April 2013, which is that elements with dir=auto (and <bdi>
without dir) that contain no strong LTR or strong RTL characters fall
back to ltr, rather than inheriting the parent's directionality.

This matches what is implemented in Gecko and WebKit, and what was
implemented in Chromium until a few months ago. A small number of
regressions were reported as a result of this change. Given that I
don't see a clear consensus that the spec behavior is better, it seems
better to revert back to the existing behavior and avoid breaking some
existing content that depends on it.

Fixes #10097.

source

@@ -13605,8 +13605,8 @@ Transport Protocol">HTTP&lt;/abbr> today.&lt;/p></code></pre> <!-- DO NOT REWRAP
      <li><p>Let <var>result</var> be the <span>auto directionality</span> of
      <var>element</var>.</p></li>
 
-     <li><p>If <var>result</var> is null, then return the <span>parent directionality</span> of
-     <var>element</var>.</p></li>
+     <li><p>If <var>result</var> is null, then return '<span
+     data-x="concept-ltr">ltr</span>'.</p></li>
 
      <li><p>Return <var>result</var>.</p></li>
     </ol>
@@ -13615,9 +13615,18 @@ Transport Protocol">HTTP&lt;/abbr> today.&lt;/p></code></pre> <!-- DO NOT REWRAP
    <dt><span data-x="attr-dir-undefined-state">undefined</span></dt>
    <dd>
     <dl class="switch">
-     <dt>If <var>element</var> is a <code>bdi</code> element and the <span>auto
-     directionality</span> of <var>element</var> is not null</dt>
-     <dd><p>Return the <span>auto directionality</span> of <var>element</var>.</p></dd>
+     <dt>If <var>element</var> is a <code>bdi</code> element</dt>
+     <dd>
+      <ol>
+       <li><p>Let <var>result</var> be the <span>auto directionality</span> of
+       <var>element</var>.</p></li>
+
+       <li><p>If <var>result</var> is null, then return '<span
+       data-x="concept-ltr">ltr</span>'.</p></li>
+
+       <li><p>Return <var>result</var>.</p></li>
+      </ol>
+     </dd>
 
      <dt>If <var>element</var> is an <code>input</code> element whose <code
      data-x="attr-input-type">type</code> attribute is in the <span