Move the notion of current Tab from TabModel to WebStateList.

ios/chrome/browser/metrics/BUILD.gn

Index: ios/chrome/browser/metrics/BUILD.gn
diff --git a/ios/chrome/browser/metrics/BUILD.gn b/ios/chrome/browser/metrics/BUILD.gn
index 93a5c4841b2d95e9048a037501518d5dd26ce8ea..b89c71013cc5379865f1a1ff6f31bf0ca2437396 100644
--- a/ios/chrome/browser/metrics/BUILD.gn
+++ b/ios/chrome/browser/metrics/BUILD.gn
@@ -109,9 +109,26 @@ source_set("metrics_internal") {
     "//ui/base",
     "//url",
   ]
+  public_deps = [
+    ":metrics_internal_arc",
+  ]
+  allow_circular_includes_from = [ ":metrics_internal_arc" ]
   libs = [ "UIKit.framework" ]
 }
 
+source_set("metrics_internal_arc") {
+  sources = [
+    "tab_usage_recorder_web_state_list_observer.h",
+    "tab_usage_recorder_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_internal") {
   testonly = true
   sources = [