Move the notion of current Tab from TabModel to WebStateList.

ios/chrome/browser/snapshots/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("snapshots") {
   sources = [
     "lru_cache.h",
     "lru_cache.mm",
     "snapshot_cache.h",
     "snapshot_cache.mm",
     "snapshot_cache_internal.h",
     "snapshot_manager.h",
     "snapshot_manager.mm",
     "snapshot_overlay.h",
     "snapshot_overlay.mm",
     "snapshots_util.h",
     "snapshots_util.mm",
   ]
   deps = [
     "//base",
     "//ios/chrome/browser",
     "//ios/chrome/browser/ui",
     "//ios/web",
   ]
+  public_deps = [
+    ":snapshots_arc",
+  ]
+  allow_circular_includes_from = [ ":snapshots_arc" ]
   libs = [ "QuartzCore.framework" ]
 }
 
+source_set("snapshots_arc") {
+  sources = [
+    "snapshot_cache_web_state_list_observer.h",
+    "snapshot_cache_web_state_list_observer.mm",
+  ]
+  deps = [
+    "//base",
+    "//ios/chrome/browser/tabs",
+    "//ios/shared/chrome/browser/tabs",
+  ]
+  configs += [ "//build/config/compiler:enable_arc" ]
+}
+
 source_set("unit_tests") {
   testonly = true
   sources = [
     "lru_cache_unittest.mm",
     "snapshot_cache_unittest.mm",
     "snapshots_util_unittest.mm",
   ]
   deps = [
     ":snapshots",
     "//base",
@@ skipping 12 matching lines @@
   ]
   deps = [
     ":snapshots",
     "//base",
     "//ios/chrome/browser/tabs",
     "//ios/chrome/browser/ui",
     "//ios/web",
   ]
   libs = [ "UIKit.framework" ]
 }