overview: Fix crash when exit after using Ctrl Shift U.

We should instead prevent Ctrl Shift U mode in overview somehow, but this
will stop the crash in the meantime.

Test: ash_unittests WindowSelectorTest.ExitInUnderlineMode
Bug: 807517
Change-Id: Ie7d889937c110583454a281a318b682f62cd6084
Reviewed-on: https://chromium-review.googlesource.com/932547Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#540664}

ash/wm/overview/window_selector.cc

@@ -757,6 +757,12 @@ void WindowSelector::OnAttemptToReactivateWindow(aura::Window* request_active,
 
 void WindowSelector::ContentsChanged(views::Textfield* sender,
                                      const base::string16& new_contents) {
+  // If the user enters underline mode via CTRL+SHIFT+U, ContentsChanged
+  // will get called after shutdown has started. Prevent anything from
+  // happening if shutdown has started (grids have been cleared).
+  if (grid_list_.size() < 1)
+    return;
+
   text_filter_string_length_ = new_contents.length();
   if (!text_filter_string_length_)
     num_times_textfield_cleared_++;

ash/wm/overview/window_selector_unittest.cc

@@ -2713,6 +2713,25 @@ TEST_F(WindowSelectorTest, Backdrop) {
   ToggleOverview();
 }
 
+// Verify that the system does not crash when exiting overview mode after
+// pressing CTRL+SHIFT+U.
+TEST_F(WindowSelectorTest, ExitInUnderlineMode) {
+  std::unique_ptr<aura::Window> window(
+      CreateWindow(gfx::Rect(10, 10, 200, 200)));
+
+  ToggleOverview();
+
+  // Enter underline mode on the text selector by generating CTRL+SHIFT+U
+  // sequence.
+  GetEventGenerator().PressKey(ui::VKEY_U,
+                               ui::EF_CONTROL_DOWN | ui::EF_SHIFT_DOWN);
+  GetEventGenerator().ReleaseKey(ui::VKEY_U,
+                                 ui::EF_CONTROL_DOWN | ui::EF_SHIFT_DOWN);
+
+  // Test that leaving overview mode cleans up properly.
+  ToggleOverview();
+}
+
 class SplitViewWindowSelectorTest : public WindowSelectorTest {
  public:
   SplitViewWindowSelectorTest() = default;