Removed property object from make_computed_style_base.py templates

Removed references to the property object from templates used in
make_computed_style_base.py. This is important pre-work for adding
fields that don't have a property object associated with them, but even
if those aren't added, reducing the amount of information sent to the
templates makes the code easier to follow and understand.

BUG=628043

Review-Url: https://codereview.chromium.org/2554243002
Cr-Commit-Position: refs/heads/master@{#437456}
3 files changed