[ios] Adds code to support the iOS Handoff feature.

ios/chrome/browser/device_sharing/device_sharing_manager.mm

+// 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.
+
+#include "ios/chrome/browser/device_sharing/device_sharing_manager.h"
+
+#include <memory>
+
+#include "base/mac/scoped_nsobject.h"
+#import "components/handoff/handoff_manager.h"
+#include "components/prefs/pref_change_registrar.h"
+#include "components/prefs/pref_service.h"
+#include "ios/chrome/browser/browser_state/chrome_browser_state.h"
+#include "ios/chrome/browser/pref_names.h"
+#include "ios/chrome/browser/prefs/pref_observer_bridge.h"
+#include "url/gurl.h"
+
+@interface DeviceSharingManager ()<PrefObserverDelegate> {
+  ios::ChromeBrowserState* _browserState;  // weak
+
+  // Bridge to listen to pref changes to the active browser state.
+  std::unique_ptr<PrefObserverBridge> _browserStatePrefObserverBridge;
+
+  // Registrar for pref change notifications to the active browser state.
+  std::unique_ptr<PrefChangeRegistrar> _browserStatePrefChangeRegistrar;
+
+  // Responsible for maintaining all state related to the Handoff feature.
+  base::scoped_nsobject<HandoffManager> _handoffManager;
+}
+
+// If handoff is enabled for the active browser state, then this method ensures
+// that a handoff manager is active. Destroys the handoff manager otherwise.
+- (void)updateHandoffManager;
+
+// Returns an auto-released instance of |HandoffManager|. Factory method
+// required for unit testing.
++ (HandoffManager*)createHandoffManager;
+
+@end
+
+@implementation DeviceSharingManager
+
+- (void)updateBrowserState:(ios::ChromeBrowserState*)state {
+  DCHECK(!state || !state->IsOffTheRecord());
+  if (_browserState == state) {
+    return;
+  }
+
+  _browserState = state;
+  if (!_browserState) {
+    _browserStatePrefObserverBridge.reset();
+    _browserStatePrefChangeRegistrar.reset();
+  } else {
+    // Add a listener to changes to the preferences related to device sharing.
+    _browserStatePrefChangeRegistrar.reset(new PrefChangeRegistrar);
+    _browserStatePrefChangeRegistrar->Init(_browserState->GetPrefs());
+    _browserStatePrefObserverBridge.reset(new PrefObserverBridge(self));
+    _browserStatePrefObserverBridge->ObserveChangesForPreference(
+        prefs::kIosHandoffToOtherDevices,
+        _browserStatePrefChangeRegistrar.get());
+  }
+  [self updateHandoffManager];
+  [self updateActiveURL:GURL()];
+}
+
+- (void)updateActiveURL:(const GURL&)activeURL {
+  [_handoffManager updateActiveURL:activeURL];
+}
+
+- (void)updateHandoffManager {
+  BOOL handoffEnabled =
+      _browserState &&
+      _browserState->GetPrefs()->GetBoolean(prefs::kIosHandoffToOtherDevices);
+  if (!handoffEnabled) {
+    _handoffManager.reset();
+    return;
+  }
+
+  if (!_handoffManager)
+    _handoffManager.reset([[[self class] createHandoffManager] retain]);
+}
+
++ (HandoffManager*)createHandoffManager {
+  return [[[HandoffManager alloc] init] autorelease];
+}
+
+#pragma mark - PrefObserverDelegate
+
+- (void)onPreferenceChanged:(const std::string&)preferenceName {
+  if (preferenceName == prefs::kIosHandoffToOtherDevices) {
+    [self updateHandoffManager];
+  }
+  // TODO(msarda): Handle nearby sharing pref changes.

[erikchen, 2016/11/08 18:13:55]

msarda is getting rid of this downstream:
https://chromereviews.googleplex.com/538247013/

[rohitrao (ping after 24h), 2016/11/08 19:57:35]

Done.

+}
+
+@end
+
+@implementation DeviceSharingManager (TestingOnly)
+
+- (HandoffManager*)handoffManager {
+  return _handoffManager.get();
+}
+
+@end