The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/2eab1a4daa484ff4e22c6a436dff7f751dab7ce0

commit 2eab1a4daa484ff4e22c6a436dff7f751dab7ce0
Author: Chris Nardi <cnardi@chromium.org>
Date: Thu Apr 19 11:56:50 2018

[cssom] Do not serialize | for a null namespace in attribute selector

According to the spec [1], | is only serialized along with the namespace
prefix if it is not the null namespace. Correct this in our
implementation, matching Firefox and an existing test in WPT.

[1]: https://drafts.csswg.org/cssom/#serialize-a-simple-selector

Bug:  834079 
Change-Id: Id137c59d3521dd4840b6d478c8c0b673f2775212
Reviewed-on: https://chromium-review.googlesource.com/1015977
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Chris Nardi <cnardi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551985}
[delete] https://crrev.com/37490077e27e172f113f253dc9e63a988eab55b4/third_party/WebKit/LayoutTests/external/wpt/css/cssom/CSSStyleRule-set-selectorText-expected.txt
[modify] https://crrev.com/2eab1a4daa484ff4e22c6a436dff7f751dab7ce0/third_party/blink/renderer/core/css/css_selector.cc