Split //ios/chrome/browser in multiple targets.

ios/chrome/browser/interstitials/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("interstitials") {
+  sources = [
+    "ios_chrome_controller_client.h",
+    "ios_chrome_controller_client.mm",
+    "ios_chrome_metrics_helper.h",
+    "ios_chrome_metrics_helper.mm",
+    "ios_security_interstitial_page.h",
+    "ios_security_interstitial_page.mm",
+  ]
+  deps = [
+    "//base",
+    "//components/history/core/browser",
+    "//components/keyed_service/core",
+    "//components/prefs",
+    "//components/rappor",
+    "//components/resources",
+    "//components/security_interstitials/core",
+    "//ios/chrome/browser:browser_no_public_deps",
+    "//ios/chrome/browser/browser_state",
+    "//ios/chrome/browser/history",
+    "//ios/web",
+    "//ui/base",
+    "//url",
+  ]
+}