Use content width to calculate percentage text-indent

The new line in TestExpectations is for an old WPT test that checks
for the old behavior.

BUG=884588
R=eae@chromium.org

Change-Id: I560e8d240d773536d084b598ac76261f9ecef81a
Reviewed-on: https://chromium-review.googlesource.com/c/1285109
Commit-Queue: Stefan Zager <szager@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600989}
7 files changed