[ObjC ARC] Converts part of ios/web:web to ARC.

Automatically generated ARCMigrate commit
Notable issues:
 - added bridged casts where necessary
 - replaced retain-autorelease with NS_VALID_UNTIL_END_OF_SCOPE
BUG=624363
TEST=None

Change-Id: Ibd7f03942d784bdcf2b95a30f693cfd736ef13c9
Reviewed-on: https://chromium-review.googlesource.com/570031
Commit-Queue: Stepan Khapugin <stkhapugin@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487047}
12 files changed