CHECK for owned_contents in TabDragController

Used to test grt@'s hypothesis in https://crbug.com/677806.

Bug: 677806
Change-Id: I1f2fa6c799fdd765f7dfbe2530f4fb5916fbb5f2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2443976
Commit-Queue: Peter Boström <pbos@chromium.org>
Commit-Queue: Connie Wan <connily@chromium.org>
Auto-Submit: Peter Boström <pbos@chromium.org>
Reviewed-by: Connie Wan <connily@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813016}

chrome/browser/ui/views/tabs/tab_drag_controller.cc

@@ -1216,6 +1216,9 @@ void TabDragController::Attach(TabDragContext* attached_context,
       if (drag_data_[i].pinned)
         add_types |= TabStripModel::ADD_PINNED;
 
+      // We should have owned_contents here, this CHECK is used to gather data
+      // for https://crbug.com/677806.
+      CHECK(drag_data_[i].owned_contents);
       attached_context_->GetTabStripModel()->InsertWebContentsAt(
           index + i - first_tab_index(),
           std::move(drag_data_[i].owned_contents), add_types, group_);
@@ -1755,6 +1758,9 @@ void TabDragController::CompleteDrag() {
     std::vector<TabStripModelDelegate::NewStripContents> contentses;
     for (size_t i = 0; i < drag_data_.size(); ++i) {
       TabStripModelDelegate::NewStripContents item;
+      // We should have owned_contents here, this CHECK is used to gather data
+      // for https://crbug.com/677806.
+      CHECK(drag_data_[i].owned_contents);
       item.web_contents = std::move(drag_data_[i].owned_contents);
       item.add_types = drag_data_[i].pinned ? TabStripModel::ADD_PINNED
                                             : TabStripModel::ADD_NONE;