DevTools: fix DOM.getContentQuads for a corner case

This patch fixes DOM.getContentQuads for a case when inline
element has only a single ::before element which is also inline.

BUG=952562
R=dgozman,pfeldman

Change-Id: I76e1c6cca468169b17cbe0662b263dd70936ad7b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1564937
Reviewed-by: Pavel Feldman <pfeldman@chromium.org>
Commit-Queue: Andrey Lushnikov <lushnikov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#650583}
3 files changed