events: Make it difficult to set the wrong flag.

Validate that the |changed_button_flags_| field is allowed only the
set of flags that makes sense.

BUG=779372, 824629

Change-Id: I5e4d8b8f07fbf6d881d8510f45414814d074278d
Reviewed-on: https://chromium-review.googlesource.com/974078
Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545120}

ash/drag_drop/drag_drop_tracker.cc

@@ -85,10 +85,12 @@ ui::LocatedEvent* DragDropTracker::ConvertEvent(aura::Window* target,
   aura::Window::ConvertPointToTarget(capture_window_->GetRootWindow(),
                                      wm::GetRootWindowAt(location_in_screen),
                                      &target_root_location);
-  return new ui::MouseEvent(
-      event.type(), target_location, target_root_location,
-      ui::EventTimeForNow(), event.flags(),
-      static_cast<const ui::MouseEvent&>(event).changed_button_flags());
+  int changed_button_flags = 0;
+  if (event.IsMouseEvent())
+    changed_button_flags = event.AsMouseEvent()->changed_button_flags();
+  return new ui::MouseEvent(event.type(), target_location, target_root_location,
+                            ui::EventTimeForNow(), event.flags(),
+                            changed_button_flags);
 }
 
 }  // namespace ash

ui/events/event.cc

@@ -198,6 +198,11 @@ bool X11EventHasNonStandardState(const base::NativeEvent& event) {
 }
 #endif
 
+constexpr int kChangedButtonFlagMask =
+    ui::EF_LEFT_MOUSE_BUTTON | ui::EF_MIDDLE_MOUSE_BUTTON |
+    ui::EF_RIGHT_MOUSE_BUTTON | ui::EF_BACK_MOUSE_BUTTON |
+    ui::EF_FORWARD_MOUSE_BUTTON;
+
 }  // namespace
 
 ////////////////////////////////////////////////////////////////////////////////
@@ -624,6 +629,8 @@ MouseEvent::MouseEvent(EventType type,
       changed_button_flags_(changed_button_flags),
       pointer_details_(pointer_details) {
   DCHECK_NE(ET_MOUSEWHEEL, type);
+  DCHECK_EQ(changed_button_flags_,
+            changed_button_flags_ & kChangedButtonFlagMask);
   latency()->set_source_event_type(ui::SourceEventType::MOUSE);
   latency()->AddLatencyNumber(INPUT_EVENT_LATENCY_UI_COMPONENT, 0, 0);
   if (this->type() == ET_MOUSE_MOVED && IsAnyButton())

ui/views/controls/label_unittest.cc

@@ -193,7 +193,7 @@ class LabelSelectionTest : public LabelTest {
   void PerformMousePress(const gfx::Point& point, int extra_flags = 0) {
     ui::MouseEvent pressed_event = ui::MouseEvent(
         ui::ET_MOUSE_PRESSED, point, point, ui::EventTimeForNow(),
-        ui::EF_LEFT_MOUSE_BUTTON, ui::EF_LEFT_MOUSE_BUTTON | extra_flags);
+        ui::EF_LEFT_MOUSE_BUTTON, ui::EF_LEFT_MOUSE_BUTTON);
     label()->OnMousePressed(pressed_event);
   }
 

ui/views/controls/textfield/textfield_unittest.cc

@@ -731,7 +731,7 @@ class TextfieldTest : public ViewsTestBase, public TextfieldController {
   void PressMouseButton(ui::EventFlags mouse_button_flags, int extra_flags) {
     ui::MouseEvent press(ui::ET_MOUSE_PRESSED, mouse_position_, mouse_position_,
                          ui::EventTimeForNow(), mouse_button_flags,
-                         mouse_button_flags | extra_flags);
+                         mouse_button_flags);
     textfield_->OnMousePressed(press);
   }