[css-flexbox] Update assert and issue number of percentage-size-quirks-001.html

This CL is a follow-up CL of crrev.com/c/2134466. This TC has an
issue number[1] that introduced it. But, since Chromium changed
the behavior at [2], it should be updated with the latter. It also
updates "assert" to deliver clear messages.

[1]https://bugs.webkit.org/show_bug.cgi?id=81809
[2]crbug.com/531783

Bug: 1063749
Change-Id: If3b1c761cab29e351e337eda4f3797f12679140c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2143094
Commit-Queue: Julie Kim <jkim@igalia.com>
Reviewed-by: David Grogan <dgrogan@chromium.org>
Reviewed-by: Robert Ma <robertma@chromium.org>
Cr-Commit-Position: refs/heads/master@{#758144}

third_party/blink/web_tests/external/wpt/css/css-flexbox/percentage-size-quirks-001.html → third_party/blink/web_tests/external/wpt/css/css-flexbox/percentage-size-quirks.html

 <title>CSS Flexbox: percentage size in flexbox children in quirks mode</title>
 <link rel="help" href="https://drafts.csswg.org/css-flexbox/#flex-containers">
 <link rel="help" href="https://drafts.csswg.org/css-sizing-3/#percentage-sizing">
-<link rel="help" href="https://bugs.webkit.org/show_bug.cgi?id=81809">
-<meta name="assert" content="This test checks that percentage height/width values in quirks mode are correctly resolved in flexbox children.">
+<link rel="help" href="https://crbug.com/531783">
+<meta name="assert" content="This test checks that percentage height/width values in flexbox children are correctly resolved regardless of quirks mode.">
 <style>
 .flexbox {
     display: flex;