Rename OPTIONAL_ACCESSOR to OPTIONAL_ACCESSORS (to be in line with
ACCESSORS and DECL_OPTIONAL_ACCESSORS), and make OPTIONAL_ACCESSORS also
define the ACCESSORS (to be in line with DECL_OPTIONAL_ACCESSORS).

Drive-by: Remove DECL_OPTIONAL_GETTER; just define the setter too.

R=titzer@chromium.org

Change-Id: Id41a652153d7a1e2a15a5f69c6f52b96298f9dc1
Reviewed-on: https://chromium-review.googlesource.com/598868Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47111}