Expose thumbnails of pages to iOS share extensions.

ios/chrome/browser/ui/activity_services/chrome_activity_item_thumbnail_generator.mm

Index: ios/chrome/browser/ui/activity_services/chrome_activity_item_thumbnail_generator.mm
diff --git a/ios/chrome/browser/ui/activity_services/chrome_activity_item_thumbnail_generator.mm b/ios/chrome/browser/ui/activity_services/chrome_activity_item_thumbnail_generator.mm
new file mode 100644
index 0000000000000000000000000000000000000000..15cf53164e005d94fe0c2fcb8b75809f5f8c37c2
--- /dev/null
+++ b/ios/chrome/browser/ui/activity_services/chrome_activity_item_thumbnail_generator.mm
@@ -0,0 +1,26 @@
+// Copyright 2017 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.
+
+#import "ios/chrome/browser/ui/activity_services/chrome_activity_item_thumbnail_generator.h"
+
+#import "ios/chrome/browser/tabs/tab.h"
+
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
+namespace activity_services {
+
+ThumbnailGenerator thumbNailGeneratorForTab(Tab* tab) {
+  __weak Tab* weakTab = tab;

[Olivier, 2017/01/20 10:14:00]

This is C++. Use weak_tab

[jif, 2017/01/24 10:29:50]

But this is Obj-C++.

+  ThumbnailGenerator generator = ^UIImage*(CGSize size) {
+    Tab* strongTab = weakTab;

[Olivier, 2017/01/20 10:14:00]

nit: Is weaktab needed?
Will self not be retained on call on next line?

[jif, 2017/01/24 10:29:50]

I don't understand, there is no self here.

+    UIImage* pagePreview =
+        [strongTab generateSnapshotWithOverlay:NO visibleFrameOnly:YES];

[Olivier, 2017/01/20 20:41:59]

Is the size used?

[jif, 2017/01/24 10:29:50]

Done.

+    return pagePreview;
+  };
+  return generator;
+}
+
+}  // namespace activity_services