Replace temporary unsafe_unretained with weak.

Now that everything in ios/ is built with ARC, it's possible to use
__weak everywhere instead of unsafe_unretained, except for where the
API dictates otherwise, especially:
1. countByEnumeratingWithState:objects:count:
2. getArgument:atIndex:

Bug: 717540
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I700f0ce62f7fd5801c4ed2e9bbfed81065b0bf0f
Reviewed-on: https://chromium-review.googlesource.com/845686
Commit-Queue: Stepan Khapugin <stkhapugin@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#526968}
4 files changed