Handle initial values in background-repeat introduced by the background shorthand

background-repeat serialization did not handle initial keyword values in
background-repeat value lists.
Initial values in value lists are introduced by setting the background
shorthand to multiple background values while omitting background-repeat
properties.
Example: background: url(#1), url(#2), url(#3);
The background-repeat gets stored internally as "initial, initial, initial".

This patch updates StylePropertySerializer::backgroundRepeatPropertyValue()
to handle this case correctly.

BUG=378167

Review URL: https://codereview.chromium.org/303993003

git-svn-id: svn://svn.chromium.org/blink/trunk@175261 bbb929c8-8fbe-4397-9dbb-9b2b20218538