Make dir=auto fall back to the parent's directionality rather than ltr.

When dir=auto fails to find text with strong directionality (or a <slot>
element) to determine the directionality of the element, it should fall
back to the directionality that it would have inherited from its parent
(or, for <slot>, its shadow host) rather than falling back to ltr.

This requires updating directionality invalidation to account for the
possibility that elements with dir=auto can inherit directionality.

This is based on the proposed behavior described in:
whatwg/html#3699 (comment)
which is in the process of being specified in:
whatwg/html#9554
whatwg/html#9796

This is one of two changes needed to fix the failure of:
external/wpt/html/dom/elements/global-attributes/dir-shadow-41.html
in the still-unlanded WPT PR at
#29820

Bug: 576815
Change-Id: I9fc7c85875074ad41704ab45ec70c7632c3f8d31

html/dom/elements/global-attributes/dir_auto-N-EN-ref.html

@@ -10,7 +10,7 @@
     <meta name="assert" content="
       When dir='auto', the direction is set according to the first strong character
       of the text, ignoring neutrals and numbers.
-      If there is no strong character, as in this test, the direction defaults to LTR." />
+      If there is no strong character, as in this test, the direction defaults to the parent." />
     <style>
       input, textarea {
         font-size:1em;
@@ -35,15 +35,15 @@
         <p dir="ltr">@123!</p>
       </div>
       <div dir="rtl">
-        <p dir="ltr">@123!</p>
+        <p dir="rtl">@123!</p>
       </div>
     </div>
     <div class="ref">
       <div dir="ltr">
         <p dir="ltr">@123!</p>
       </div>
       <div dir="rtl">
-        <p dir="ltr">@123!</p>
+        <p dir="rtl">@123!</p>
       </div>
     </div>
   </body>

html/dom/elements/global-attributes/dir_auto-N-EN.html

@@ -11,7 +11,7 @@
     <meta name="assert" content="
       When dir='auto', the direction is set according to the first strong character
       of the text, ignoring neutrals and numbers.
-      If there is no strong character, as in this test, the direction defaults to LTR." />
+      If there is no strong character, as in this test, the direction defaults to the parent." />
     <style>
       input, textarea {
         font-size:1em;
@@ -44,7 +44,7 @@
         <p dir="ltr">@123!</p>
       </div>
       <div dir="rtl">
-        <p dir="ltr">@123!</p>
+        <p dir="rtl">@123!</p>
       </div>
     </div>
   </body>