URL: trim leading slashes of file URL paths

Tests: web-platform-tests/wpt#5195.

Closes #234 by superseding it. Fixes #232.

Author: annevk

Diff for: url.bs

@@ -2113,6 +2113,12 @@ string <var>input</var>, optionally with a <a>base URL</a> <var>base</var>, opti
 
        <li><p>Set <var>buffer</var> to the empty string.
 
+       <li><p>If <var>url</var>'s <a for=url>scheme</a> is "<code>file</code>" and <a>c</a> is
+       <a>EOF code point</a>, "<code>?</code>", or "<code>#</code>", then while <var>url</var>'s
+       <a for=url>path</a>'s <a for=list>size</a> is not 1 and <var>url</var>'s
+       <a for=url>path</a>[0] is the empty string, <a>validation error</a>, <a for=list>remove</a>
+       <var>url</var>'s <a for=url>path</a>[0].
+
        <li><p>If <a>c</a> is "<code>?</code>", set
        <var>url</var>'s <a for=url>query</a> to the empty string,
        and <var>state</var> to <a>query state</a>.