[ios] Convert //ios/chrome/browser/tabs:unit_tests to ARC.

ios/chrome/browser/tabs/BUILD.gn

Index: ios/chrome/browser/tabs/BUILD.gn
diff --git a/ios/chrome/browser/tabs/BUILD.gn b/ios/chrome/browser/tabs/BUILD.gn
index 51a127842f2156478d204247086a29c229d74ed6..c73b2c3251bbc93f2adf003a7ffd89330938a1a2 100644
--- a/ios/chrome/browser/tabs/BUILD.gn
+++ b/ios/chrome/browser/tabs/BUILD.gn
@@ -142,13 +142,13 @@ source_set("tabs_internal") {
 source_set("unit_tests") {
   testonly = true
   sources = [
+    "tab_model_list_unittest.mm",
     "tab_model_unittest.mm",
     "tab_unittest.mm",
   ]
   deps = [
     ":tabs",
     ":tabs_internal",
-    ":unit_tests_arc",
     "//base",
     "//components/bookmarks/test",
     "//components/history/core/browser",
@@ -175,21 +175,5 @@ source_set("unit_tests") {
     "//testing/gtest",
     "//third_party/ocmock",
   ]
-}
-
-source_set("unit_tests_arc") {
-  testonly = true
-  sources = [
-    "tab_model_list_unittest.mm",
-  ]
-  deps = [
-    ":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" ]
 }