Generate accessors for WillChange{Contents, Position, Properties}.

This patch changes the following storage_only fields to have a different
field template:
- WillChangeContents -> primitive
- WillChangePosition -> primitive
- WillChangeProperties -> external

This generates accessors for these three fields.

Diff of generated files:
https://gist.github.com/3dace3c035b90f4b096055ecbe1be3e3/revisions

Bug: 628043
Change-Id: I4e503183ca10196103ff51d539aeed7711e48354
Reviewed-on: https://chromium-review.googlesource.com/536617
Reviewed-by: nainar <nainar@chromium.org>
Commit-Queue: Darren Shen <shend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#479945}
2 files changed