Add support for 'x' as a unit of resolution

In https://drafts.csswg.org/css-values-4/#resolution, 'x' is defined as
a synonym for dppx. Add support for parsing 'x' as dots per px.

Intent to Implement and Ship:
https://groups.google.com/a/chromium.org/d/topic/blink-dev/FsD97U77tss

Bug: 630597, 807653
Change-Id: I70c440c89e42cd005a9c58fd89745a5603e0dee6
Reviewed-on: https://chromium-review.googlesource.com/892467
Commit-Queue: Chris Nardi <cnardi@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558119}
4 files changed