Empty SVG*List should serialize to the null String

When synchronizing an empty list, the result should be that the
attribute is removed. However, since StringBuilder::ToString for an
"empty" StringBuilder returns the empty string we end up setting an
empty attribute. Add a special case to handle empty lists, and return
the null String there instead. The null String will translate to an
attribute removal.

All the SVG*List types that inherit from SVGListPropertyHelper are
changed call a new shared implementation (SerializeList) in that class.

Bug: 843901
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I9442268196aaa605886fd4d6d31d66ae075f901d
Reviewed-on: https://chromium-review.googlesource.com/1189862
Reviewed-by: Stephen Chenney <schenney@chromium.org>
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Cr-Commit-Position: refs/heads/master@{#586251}
7 files changed