Make <hr> have 'overflow: hidden' by default

This causes the element to create a block formatting context, which
means it will avoid floats and grow to contain floats. This matches
EdgeHTML. WebKit/Chromium/Gecko avoid floats but do not create a
block formatting context, which is weird.

Fixes whatwg#2715.

Tests: web-platform-tests/wpt#6115

source

@@ -111574,6 +111574,7 @@ hr {
   margin-inline-end: auto;
   margin-block-end: 0.5em;
   margin-inline-start: auto;
+  overflow: hidden;
 }</pre>
 
   <p>The following rules are also expected to apply, as <span>presentational hints</span>:</p>
@@ -121150,6 +121151,7 @@ INSERT INTERFACES HERE
   Mohamed Zergaoui<!-- Innovimax SARL -->,
   Mohammad Al Houssami,
   Momdo Nakamura,
+  Morten Stenshorne,
   Mounir Lamouri,
   Ms2ger,
   Nadia Heninger,