[ObjC ARC] Converts components/signin/ios/browser:browser to ARC.

Automatically generated ARCMigrate commit

Notable issues: Observer bridge classes need to explicitly declare their delegates weak.

BUG=624363
TEST=None

Review-Url: https://codereview.chromium.org/2928383002
Cr-Commit-Position: refs/heads/master@{#479670}
9 files changed