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..ff196620aeda88ecf4ce751bb2ff1731b3fc7b9d
--- /dev/null
+++ b/ios/chrome/browser/ui/activity_services/chrome_activity_item_thumbnail_generator.mm
@@ -0,0 +1,33 @@
+// 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) {
+  __weak Tab* weakTab = tab;
+  ThumbnailGeneratorBlock generator = ^UIImage*(CGSize const& size) {
+    Tab* strongTab = weakTab;
+    // Do not generate thumbnails for incognito tabs.
+    if (strongTab.browserState->IsOffTheRecord())

[Olivier, 2017/01/24 16:54:05]

browser state is a C++ object.
If strongTab is nil, browserState will be null and you get a null dereference
exception.

[Olivier, 2017/01/24 16:54:05]

nit: I don't think tab can change between incognito and not incognito.
Can you test this before and not give a ThumbnailGeneratorBlock for incognito
tabs?

[jif, 2017/01/25 15:17:29]

oops

[jif, 2017/01/25 15:17:29]

Ack.
Because I don't like returning nil blocks, I'll return a block that does
nothing.

+      return nil;
+    UIImage* snapshot =
+        [strongTab generateSnapshotWithOverlay:NO visibleFrameOnly:YES];
+    UIImage* thumbnail =
+        ResizeImage(snapshot, size, ProjectionMode::kAspectFillAlignTop, YES);
+    return thumbnail;
+  };
+  return generator;
+}
+
+}  // namespace activity_services