[css-properties-values-api] Support StylePropertyMap.append.

This CL makes it possible to append values to list-valued properties.

To do this, we must produce a CSSStyleValueVector which holds the final
value produced by the append (i.e. the concatenation of the existing
values and the incoming values). Then (like for StylePropertyMap.set),
a CSSVariableReferenceValue with equivalent tokens is created from that
vector.

Bonus: Fix names of some tests to make it easier to read the output.

R=chrishtr@chromium.org

Bug: 641877
Change-Id: I0d58f66255a9ff7e14c1ee8f4d243353b9a4a0da
Reviewed-on: https://chromium-review.googlesource.com/c/1270967
Commit-Queue: Anders Ruud <andruud@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599177}
4 files changed