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..51dcb3eeef0250e6f97885f18aa8be1f19bb8fef
--- /dev/null
+++ b/ios/chrome/browser/ui/activity_services/chrome_activity_item_thumbnail_generator.mm
@@ -0,0 +1,37 @@
+// 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"
+
+#include "ios/chrome/browser/browser_state/chrome_browser_state.h"
+#import "ios/chrome/browser/tabs/tab.h"
+#import "ios/chrome/browser/ui/uikit_ui_util.h"
+
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
+namespace activity_services {
+
+ThumbnailGeneratorBlock ThumbnailGeneratorForTab(Tab* tab) {
+  DCHECK(tab);
+  // Do not generate thumbnails for incognito tabs.
+  if (tab.browserState->IsOffTheRecord()) {
+    return ^UIImage*(CGSize const& size) { return nil; };
+  } else {
+    __weak Tab* weakTab = tab;
+    return ^UIImage*(CGSize const& size) {
+      Tab* strongTab = weakTab;
+      if (!strongTab)
+        return nil;
+      UIImage* snapshot =
+          [strongTab generateSnapshotWithOverlay:NO visibleFrameOnly:YES];

[marq (ping after 24h), 2017/01/26 16:28:09]

Nit: You can just call this method on weakTab here; nil objects return nil, and
ResizeImage(nil, ...) should also be nil.

[jif, 2017/01/27 11:56:11]

The pattern of calling a method on a weak object is dangerous because the object
could released on a separate thread.

In this particular case, the tab can only be released on the UI-thread, but
that's a detail this function shouldn't know about.

[marq (ping after 24h), 2017/01/27 12:52:56]

You are of course correct.

You can (and should), however, remove the nil check on |strongTab| --
https://google.github.io/styleguide/objcguide.xml?showone=nil_Checks#nil_Checks.

[jif, 2017/01/27 13:51:54]

Done.

+      UIImage* thumbnail =
+          ResizeImage(snapshot, size, ProjectionMode::kAspectFillAlignTop, YES);
+      return thumbnail;
+    };
+  }
+}
+
+}  // namespace activity_services