[ObjC ARC] Converts ios/chrome/app/spotlight:spotlight to ARC.

Automatically generated ARCMigrate commit
Notable issues:
* in topsites_spotlight_manager.mm, added a strongself to avoid dereferncing a weak
 pointer (which would cause crashes, but is not a compiler-diagnosed error without
 ARC).

BUG=624363
TEST=None

Review-Url: https://codereview.chromium.org/2704193002
Cr-Commit-Position: refs/heads/master@{#451650}
8 files changed