[ObjC ARC] Converts a part of ios/chrome/browser/ui/bookmarks:bookmarks to ARC.

Automatically generated ARCMigrate commit (after manually splitting the target).

Converted a relevant unit test alongside this to prevent fiddling with BOOL GetPositionCache(bookmarks::BookmarkModel* model, BookmarkMenuItem** item,  CGFloat* position);
which has an out paramteter |item| that is __autoreleasing in ARC.

Notable issues:None
BUG=624363
TEST=None

Review-Url: https://codereview.chromium.org/2737793003
Cr-Commit-Position: refs/heads/master@{#456054}
6 files changed