Clear pending stylesheet flag when xml-stylesheet loads

When an xml-stylesheet is declared before the <html> element, the
document's pending stylesheet count is incremented; but the count is
not decremented when the xml-stylesheet finishes loading.

This has no effect when UpdateStyleAndLayoutIgnorePendingStylesheets
is used; but when that mechanism is removed, it blocks rendering,
causing the test named below to fail.

BUG=521692
TEST=fast/dom/StyleSheet/detached-style-pi-2.xhtml

Change-Id: Ibadae4b66d5351cd58ed00ad654247f53fba6861
Reviewed-on: https://chromium-review.googlesource.com/c/1334946
Commit-Queue: Stefan Zager <szager@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608204}
2 files changed