[ObjC ARC] Converts components/sessions:unit_tests to ARC.

"ARC" is Automatic Reference Counting, a memory management model for Objective-C.

Automatically generated ARCMigrate commit
Notable issues:None
BUG=624363
TEST=None

Review-Url: https://codereview.chromium.org/2937493002
Cr-Commit-Position: refs/heads/master@{#479648}
2 files changed