Generate getters/setters for some fields on groups in ComputedStyle

This patch generates getters/setters as needed for monotic_flags,
external and storage_only fields on groups in ComputedStyle. In a
following patch we will use this to access members of groups through
these getters/setters instead of directly as <group_name>.<field_name>.

Generated diff here: https://gist.github.com/nainar/bf6ca12c12ff7283d90ebed7640cf7be/revisions

Please note these functions are unused for now.

BUG=710938

Review-Url: https://codereview.chromium.org/2826653002
Cr-Commit-Position: refs/heads/master@{#465919}
9 files changed