[ObjC ARC] Converts ios/chrome/browser/ui/omnibox:omnibox_internal to ARC.

Automatically generated ARCMigrate commit
Notable issues:
* added some __bridge casts
* put some __weak and __unsafe_unretained classifiers in headers

BUG=624363
TEST=None

Review-Url: https://codereview.chromium.org/2707963002
Cr-Commit-Position: refs/heads/master@{#459107}
16 files changed