Fix DCHECK on attempt to crop blank thumbnail

ThumbnailImage::CropPreviewImage relies on the supplied thumbnail
having non-null image data. A null thumbnail was passed when ClearData
was called.

It is not necessary to crop a null thumbnail, so avoid calling it when
the thumbnail is null.

Fixed: 1168483
Change-Id: I89de21fb69f49069c44db54f4e82f80c3afa7da8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2641156Reviewed-by: Dana Fried <dfried@chromium.org>
Commit-Queue: Dana Fried <dfried@chromium.org>
Auto-Submit: Collin Baker <collinbaker@chromium.org>
Cr-Commit-Position: refs/heads/master@{#845590}

chrome/browser/ui/thumbnails/thumbnail_image.cc

@@ -181,9 +181,12 @@ void ThumbnailImage::NotifyUncompressedDataObservers(base::Token thumbnail_id,
 
   for (Subscription* subscription : subscribers_) {
     auto size_hint = subscription->size_hint_;
-    if (subscription->uncompressed_image_callback_)
-      subscription->uncompressed_image_callback_.Run(
-          size_hint ? CropPreviewImage(image, *size_hint) : image);
+    if (subscription->uncompressed_image_callback_) {
+      auto cropped_image = size_hint && !image.isNull()
+                               ? CropPreviewImage(image, *size_hint)
+                               : image;
+      subscription->uncompressed_image_callback_.Run(cropped_image);
+    }
   }
 }
 

chrome/browser/ui/thumbnails/thumbnail_image_unittest.cc

@@ -368,3 +368,29 @@ TEST_F(ThumbnailImageTest, UnsubscribeAfterDelegateDestroyed) {
   delegate.reset();
   subscription.reset();
 }
+
+// Ensures subscribers with a size hint get notified correctly on
+// thumbnail clear. Regression test for crbug.com/1168483 where
+// CropPreviewImage was called on blank thumbnails resulting in a
+// DCHECK.
+TEST_F(ThumbnailImageTest, DoesNotCropBlankThumbnails) {
+  auto image = base::MakeRefCounted<ThumbnailImage>(this);
+
+  std::unique_ptr<Subscription> subscription = image->Subscribe();
+  subscription->SetSizeHint(
+      gfx::Size(kTestBitmapWidth / 2, kTestBitmapHeight / 2));
+
+  CallbackWaiter uncompressed_image_waiter;
+  subscription->SetUncompressedImageCallback(
+      IgnoreArgs<gfx::ImageSkia>(uncompressed_image_waiter.callback()));
+
+  SkBitmap bitmap = CreateBitmap(kTestBitmapWidth, kTestBitmapHeight);
+  image->AssignSkBitmap(bitmap, base::nullopt);
+  uncompressed_image_waiter.Wait();
+  EXPECT_TRUE(uncompressed_image_waiter.called());
+  uncompressed_image_waiter.Reset();
+
+  image->ClearData();
+  uncompressed_image_waiter.Wait();
+  EXPECT_TRUE(uncompressed_image_waiter.called());
+}