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

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:browser_no_public_deps",
+    "//ios/chrome/browser/ui",
+    "//ios/web",
+  ]
+  libs = [ "QuartzCore.framework" ]
+}
+
+source_set("unit_tests") {
+  testonly = true
+  sources = [
+    "lru_cache_unittest.mm",
+    "snapshot_cache_unittest.mm",
+    "snapshots_util_unittest.mm",
+  ]
+  deps = [
+    ":snapshots",
+    "//base",
+    "//ios/chrome/browser",
+    "//ios/chrome/browser/ui",
+    "//ios/web",
+    "//ios/web:test_support",
+    "//testing/gtest",
+  ]
+}