cros: Handle ui::VKEY_BACK event to exit search mode in KSV.

This cl handles the ui::VKEY_BACK. If it is already empty search box and
typing ui::VKEY_BACK will exit search mode in Keyboard Shortcut
Viewer (KSV).

Bug: 768932
Test: Tested on device.
Change-Id: I5cee02a97959784163e65e300b5113f2a3409513
Reviewed-on: https://chromium-review.googlesource.com/923090
Commit-Queue: Tao Wu <wutao@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537457}

ui/chromeos/ksv/views/keyboard_shortcut_view.h

@@ -48,6 +48,8 @@ class KeyboardShortcutView : public views::WidgetDelegateView,
 
   void InitViews();
 
+  // Put focus on the active tab. Used when the first time to show the widget or
+  // after exiting search mode.
   void RequestFocusForActiveTab();
 
   static KeyboardShortcutView* GetInstanceForTesting();

ui/chromeos/ksv/views/ksv_search_box_view.cc

@@ -48,6 +48,20 @@ gfx::Size KSVSearchBoxView::CalculatePreferredSize() const {
   return gfx::Size(704, 32);
 }
 
+void KSVSearchBoxView::OnKeyEvent(ui::KeyEvent* event) {
+  if (event->key_code() != ui::VKEY_BACK)
+    return;
+
+  if (!search_box()->text().empty())
+    return;
+
+  if (exit_search_mode_on_next_backspace_) {
+    SetSearchBoxActive(false);
+    event->SetHandled();
+  }
+  exit_search_mode_on_next_backspace_ = !exit_search_mode_on_next_backspace_;
+}
+
 void KSVSearchBoxView::UpdateBackgroundColor(SkColor color) {
   SetSearchBoxBackgroundColor(color);
 }

ui/chromeos/ksv/views/ksv_search_box_view.h

@@ -22,6 +22,7 @@ class KSVSearchBoxView : public search_box::SearchBoxViewBase {
 
   // views::View:
   gfx::Size CalculatePreferredSize() const override;
+  void OnKeyEvent(ui::KeyEvent* event) override;
 
  private:
   // search_box::SearchBoxViewBase:
@@ -34,6 +35,9 @@ class KSVSearchBoxView : public search_box::SearchBoxViewBase {
   void SetupCloseButton() override;
   void SetupBackButton() override;
 
+  // True to exit search mode on the next ui::VKEY_BACK event.
+  bool exit_search_mode_on_next_backspace_ = false;
+
   DISALLOW_COPY_AND_ASSIGN(KSVSearchBoxView);
 };