Update UA stylesheet for fieldset.

Should use min-inline-size instead of min-width, so that it works as
intended in vertical writing modes too. See
https://github.com/whatwg/html/pull/3914

Bug: 874053
Change-Id: I4fd4bd74606721bb44170a926f8be3d98a67db3e
Reviewed-on: https://chromium-review.googlesource.com/1188678Reviewed-by: Emil A Eklund <eae@chromium.org>
Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585951}

third_party/WebKit/LayoutTests/external/wpt/html/rendering/non-replaced-elements/the-fieldset-element-0/min-inline-size-expected.txt

-This is a testharness.js-based test.
-PASS horizontal-tb 
-FAIL vertical-lr  assert_equals: width expected "0px" but got "100px"
-FAIL vertical-rl  assert_equals: width expected "0px" but got "100px"
-PASS horizontal-tb override
-FAIL vertical-lr override assert_equals: width expected "0px" but got "100px"
-FAIL vertical-rl override assert_equals: width expected "0px" but got "100px"
-Harness: the test ran to completion.
-

third_party/blink/renderer/core/css/html.css

@@ -390,7 +390,7 @@ fieldset {
     -webkit-padding-end: 0.75em;
     -webkit-padding-after: 0.625em;
     border: 2px groove ThreeDFace;
-    min-width: -webkit-min-content;
+    min-inline-size: min-content;
 }
 
 button {