Clean up ComputedStyle field templates.

This patch reduces code duplication by making the macros in field.tmpl
and base.tmpl more generic and granular.

- Rename argument_type(field) -> const_ref(field)
- Rename return_type(field) -> nonconst_ref(field)
- Split base.decl_public_methods into:
  - base.decl_initial_method
  - base.decl_getter_method
  - base.decl_setter_method
  - base.decl_resetter_method
- Split base.decl_protected_methods into:
  - base.decl_mutable_method
  - base.decl_internal_getter_method
  - base.decl_internal_setter_method

Diff of generated files:
https://gist.github.com/darrnshn/a4f1071f9901d5d4b321ebd166c10dfa/revisions

BUG=628043

Review-Url: https://codereview.chromium.org/2829213002
Cr-Commit-Position: refs/heads/master@{#466255}
7 files changed