[ios] Convert //ios/chrome/browser/tabs target to ARC.

ios/chrome/browser/tabs/BUILD.gn

 # Copyright 2016 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 source_set("tabs") {
   sources = [
     "legacy_tab_helper.h",
     "tab.h",

[stkhapugin, 2017/05/16 09:49:01]

This file has a bunch of properties that have no storage classificators or have
assign for objc objects
 

[sdefresne, 2017/05/16 10:06:47]

The only assign properties are for non-objc objects:
@property(nonatomic, assign) BOOL useGreyImageCache;
@property(nonatomic, assign) BOOL isPrerenderTab;
@property(nonatomic, assign) BOOL isLinkLoadingPrerenderTab;
@property(nonatomic, assign) BOOL isVoiceSearchResultsTab;

The properties without a specifier are properties that are not synthesized
(title, originalTitle, webController) or are strong (as this is the default,
like tabId, passwordController, ...).

This was done in https://codereview.chromium.org/2857133002/.

     "tab_delegate.h",
     "tab_dialog_delegate.h",
     "tab_headers_delegate.h",
     "tab_helper_util.h",
     "tab_model.h",

[stkhapugin, 2017/05/16 09:49:01]

ditto

[sdefresne, 2017/05/16 10:06:47]

Ditto, except the CL is https://codereview.chromium.org/2854133002/.

     "tab_model_list.h",
     "tab_model_observer.h",
     "tab_model_synced_window_delegate.h",

[stkhapugin, 2017/05/16 09:49:01]

This one has an ivar in a header that's marked as "// weak, owns us" and is
objc, so you need an __unsafe_unretained there

[sdefresne, 2017/05/16 10:06:47]

This no longer has any Objective-C member fields since
https://codereview.chromium.org/2845353002/.

     "tab_model_synced_window_delegate_getter.h",
     "tab_private.h",
     "tab_snapshotting_delegate.h",
   ]
   public_deps = [
     "//ios/web",
   ]
   deps = [
     "//components/sessions",
     "//components/signin/ios/browser",
     "//components/sync_sessions",
     "//ios/chrome/browser/web:sad_tab_tab_helper_delegate",
     "//ios/net",
     "//ios/web:user_agent",
     "//ui/base",
   ]
   libs = [ "UIKit.framework" ]
+  configs += [ "//build/config/compiler:enable_arc" ]
 }
 
 source_set("tabs_internal") {
   sources = [
     "legacy_tab_helper.mm",
     "tab.h",
     "tab.mm",
     "tab_helper_util.mm",
     "tab_model.mm",
     "tab_model_closing_web_state_observer.h",
@@ skipping 141 matching lines @@
     ":tabs",
     ":tabs_internal",
     "//base",
     "//ios/chrome/browser/browser_state:test_support",
     "//ios/chrome/browser/sessions:test_support",
     "//ios/web:test_support",
     "//testing/gtest",
   ]
   configs += [ "//build/config/compiler:enable_arc" ]
 }