Make focus() always center the element it scrolls into view

Previously, the inline position was implementation-defined, whereas the block position was centered. Now, they are both centered. Fixes whatwg#9518.

Also updates to use CSSOM View's new "scroll a target into view" algorithm, which replaces the broken link to "scroll an element into view" that we were previously trying to invoke.

source

@@ -3940,7 +3940,7 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
      <li><dfn data-x-href="https://drafts.csswg.org/cssom-view/#document-run-the-resize-steps">run the resize steps</dfn></li>
      <li><dfn data-x-href="https://drafts.csswg.org/cssom-view/#document-run-the-scroll-steps">run the scroll steps</dfn></li>
      <li><dfn data-x-href="https://drafts.csswg.org/cssom-view/#evaluate-media-queries-and-report-changes">evaluate media queries and report changes</dfn></li>
-     <li><dfn data-x-href="https://drafts.csswg.org/cssom-view/#scroll-an-element-into-view">Scroll an element into view</dfn></li>
+     <li><dfn data-x-href="https://drafts.csswg.org/cssom-view/#scroll-a-target-into-view">Scroll a target into view</dfn></li>
      <li><dfn data-x-href="https://drafts.csswg.org/cssom-view/#scroll-to-the-beginning-of-the-document">Scroll to the beginning of the document</dfn></li>
      <li>The <dfn data-x="event-resize" data-x-href="https://drafts.csswg.org/cssom-view/#eventdef-window-resize"><code>resize</code></dfn> event</li>
      <li>The <dfn data-x="event-scroll" data-x-href="https://drafts.csswg.org/cssom-view/#eventdef-document-scroll"><code>scroll</code></dfn> event</li>
@@ -67686,7 +67686,7 @@ try {
    <li><p>Let <var>notionalChild</var> be a hypothetical element that is a rendered child of the
    <code>canvas</code> element whose dimensions are those of <var>specifiedRectangle</var>.</p></li>
 
-   <li><p><span data-x="scroll an element into view">Scroll <var>notionalChild</var> into
+   <li><p><span data-x="scroll a target into view">Scroll <var>notionalChild</var> into
    view</span> with <var>behavior</var> set to "auto", <var>block</var> set to "start", and
    <var>inline</var> set to "nearest".</p>
 
@@ -78976,10 +78976,8 @@ partial interface <span id="NavigatorUserActivation-partial">Navigator</span> {
 
    <li><p>If the value of the <dfn dict-member for="FocusOptions"><code
    data-x="dom-focusoptions-preventscroll">preventScroll</code></dfn> dictionary member of
-   <var>options</var> is false, then <span data-x="scroll an element into view">scroll the element
-   into view</span> with scroll behavior "<code data-x="">auto</code>", block flow direction
-   position set to an <span>implementation-defined</span> value, and inline base direction position
-   set to an <span>implementation-defined</span> value.</p></li>
+   <var>options</var> is false, then <span data-x="scroll a target into view">scroll the element
+   into view</span> given "auto", "center", and "center".</p></li>
 
    <li><p>Unmark the element as <i data-x="locked for focus">locked for focus</i>.</p></li>
   </ol>
@@ -99747,7 +99745,7 @@ location.href = '#foo';</code></pre>
      <li><p>Run the <span>ancestor hidden-until-found revealing algorithm</span> on
      <var>target</var>.</p></li>
 
-     <li><p><span data-x="scroll an element into view">Scroll <var>target</var> into view</span>,
+     <li><p><span data-x="scroll a target into view">Scroll <var>target</var> into view</span>,
      with <var>behavior</var> set to "auto", <var>block</var> set to "start", and <var>inline</var>
      set to "nearest". <ref spec=CSSOMVIEW></p></li>