GitLab

 - Modify js_externs_generator_test.py to add "@typedef {Object}"
 - Mark optional parameter as optional in bookmarks_manager_private.json
 - Move incorrectly placed annotation in c/b/r/bookmarks/store.js
 - Re-generate bookmark_manager_private.js and add a note at the top of
   that file, about manual edits required.

Bug: 717828,717827
Change-Id: I69639a800d7aabb1bbe5e8885efce487f9414409
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1499778Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Auto-Submit: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#637988}