[Sync] Creating sync_sessions component, moving revisit logic.

components/sync_sessions/BUILD.gn

Index: components/sync_sessions/BUILD.gn
diff --git a/components/sync_sessions/BUILD.gn b/components/sync_sessions/BUILD.gn
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+# Copyright 2015 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.
+
+# GYP version: components/sync_sessions.gypi:sync_sessions
+source_set("sync_sessions") {
+  sources = [
+    "revisit/current_tab_matcher.cc",
+    "revisit/current_tab_matcher.h",
+    "revisit/offset_tab_matcher.cc",
+    "revisit/offset_tab_matcher.h",
+    "revisit/page_equality.h",
+    "revisit/page_visit_observer.h",
+    "revisit/sessions_page_revisit_observer.cc",
+    "revisit/sessions_page_revisit_observer.h",
+  ]
+
+  deps = [
+    "//base",
+    "//components/sessions",
+    "//components/sync_driver",
+    "//sync",
+  ]
+}
+
+source_set("unit_tests") {
+  testonly = true
+  sources = [
+    "revisit/current_tab_matcher_unittest.cc",
+    "revisit/offset_tab_matcher_unittest.cc",
+    "revisit/sessions_page_revisit_observer_unittest.cc",
+  ]
+
+  deps = [
+    ":sync_sessions",
+    "//base/test:test_support",
+    "//testing/gtest",
+    "//sync",
+  ]
+}