[ObjC ARC] Converts ios/web:ios_web_web_state_ui_unittests to ARC.

Automatically generated ARCMigrate commit

Notable issues:

Removed WKBackForwardListItemHolderTest.GetNilBackForwardListItemFromHolder,
since it amounts to being a test of WeakNSPointer, and there's no guarantees about
object lifetimes under ARC.

BUG=624363
TEST=None

Review-Url: https://codereview.chromium.org/2933363002
Cr-Commit-Position: refs/heads/master@{#479462}
10 files changed