Clears clipboard when null DataSource is passed to SetSelection

Previously SetSelection did DCHECK for null DataSrouce, which was wrong.
According to Wayland document, we should clears the clipboard when null
DataSource is passed.

Bug; 789831
Test: exo_unittests

Change-Id: I9bff52cb4259b2d02faeb6a6e32353d0e3da4f2c
Reviewed-on: https://chromium-review.googlesource.com/798815
Commit-Queue: Daichi Hirono <hirono@chromium.org>
Reviewed-by: David Reveman <reveman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520775}

components/exo/seat.cc

@@ -62,14 +62,17 @@ Surface* Seat::GetFocusedSurface() {
 }
 
 void Seat::SetSelection(DataSource* source) {
-  DCHECK(source);
+  if (!source) {
+    ui::Clipboard::GetForCurrentThread()->Clear(ui::CLIPBOARD_TYPE_COPY_PASTE);
+    // selection_source_ is Cancelled() and reset() in OnClipboardDataChanged().
+    return;
+  }
 
   if (selection_source_) {
     if (selection_source_->get() == source)
       return;
     selection_source_->get()->Cancelled();
   }
-
   selection_source_ = std::make_unique<ScopedDataSource>(source, this);
 
   // Unretained is safe as Seat always outlives DataSource.

components/exo/seat_unittest.cc

@@ -227,5 +227,26 @@ TEST_F(SeatTest, SetSelection_SourceDestroyedAfterSetSelection) {
   EXPECT_FALSE(delegate1.cancelled());
 }
 
+TEST_F(SeatTest, SetSelection_NullSource) {
+  Seat seat;
+
+  TestDataSourceDelegate delegate;
+  DataSource source(&delegate);
+  source.Offer("text/plain;charset=utf-8");
+  seat.SetSelection(&source);
+
+  RunReadingTask();
+
+  // Should clear the clipboard.
+  seat.SetSelection(nullptr);
+
+  ASSERT_TRUE(delegate.cancelled());
+
+  std::string clipboard;
+  ui::Clipboard::GetForCurrentThread()->ReadAsciiText(
+      ui::CLIPBOARD_TYPE_COPY_PASTE, &clipboard);
+  EXPECT_EQ(clipboard, "");
+}
+
 }  // namespace
 }  // namespace exo