back_gesture: Use screen location to update the affordance's position.

See video before change:
https://drive.google.com/file/d/0B5I0jFeLxqIiUlg2aXRrdGFIV0d0ZzhZYng4Y3VJQVJ1OXow/view?usp=sharing

The affordance is assumed to be 64dips above the touch point,but the
distance seems much further than expected.

See video after change:
https://drive.google.com/file/d/0B5I0jFeLxqIicGNacUIzUThxUHZ3SHV0RVBlWFpzdUNtb2h3/view?usp=sharing

Bug: 1002733
Change-Id: I4d5638a3c964fe45a52402f75c9ad21ba62a9bea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1859690
Commit-Queue: Min Chen <minch@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705993}

ash/wm/toplevel_window_event_handler.cc

@@ -863,20 +863,23 @@ bool ToplevelWindowEventHandler::HandleGoingBackFromLeftEdge(
   if (!CanStartGoingBack())
     return false;
 
+  gfx::Point screen_location = event->location();
+  ::wm::ConvertPointToScreen(static_cast<aura::Window*>(event->target()),
+                             &screen_location);
   switch (event->type()) {
     case ui::ET_GESTURE_SCROLL_BEGIN: {
       going_back_started_ = StartedAwayFromLeftArea(event);
       if (!going_back_started_)
         break;
       back_gesture_affordance_ =
-          std::make_unique<BackGestureAffordance>(event->location());
+          std::make_unique<BackGestureAffordance>(screen_location);
       return true;
     }
     case ui::ET_GESTURE_SCROLL_UPDATE:
       if (!going_back_started_)
         break;
       DCHECK(back_gesture_affordance_);
-      back_gesture_affordance_->SetDragProgress(event->location().x());
+      back_gesture_affordance_->SetDragProgress(screen_location.x());
       return true;
     case ui::ET_GESTURE_SCROLL_END:
     case ui::ET_SCROLL_FLING_START: {
@@ -884,11 +887,11 @@ bool ToplevelWindowEventHandler::HandleGoingBackFromLeftEdge(
         break;
       DCHECK(back_gesture_affordance_);
       if ((event->type() == ui::ET_GESTURE_SCROLL_END &&
-           event->location().x() >= kSwipingDistanceForGoingBack) ||
+           screen_location.x() >= kSwipingDistanceForGoingBack) ||
           (event->type() == ui::ET_SCROLL_FLING_START &&
            event->details().velocity_x() >= kFlingVelocityForGoingBack)) {
         aura::Window* root_window =
-            window_util::GetRootWindowAt(event->location());
+            window_util::GetRootWindowAt(screen_location);
         ui::KeyEvent press_key_event(ui::ET_KEY_PRESSED, ui::VKEY_BROWSER_BACK,
                                      ui::EF_NONE);
         ignore_result(