Unify logic for properties and nonproperties in computed_style_base.py

As nonproperty fields get more complex, the logic to create nonproperty
fields becomes very similar to property fields. This patch merges the
two code paths so that properties and nonproperties share as much logic
as possible.

BUG=628043

Review-Url: https://codereview.chromium.org/2766933002
Cr-Commit-Position: refs/heads/master@{#459231}
1 file changed