[exo] Register CustomFrameView as a WindowObserver.

Bug: 866370
Test: PIP window no longer has top portion cut off
Change-Id: I6a4415162fd2697b49866bbe8698d306315fe074
Reviewed-on: https://chromium-review.googlesource.com/1146524
Commit-Queue: Eliot Courtney <edcourtney@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577462}

ash/frame/custom_frame_view_ash.h

@@ -120,6 +120,8 @@ class ASH_EXPORT CustomFrameViewAsh : public views::NonClientFrameView,
   SkColor GetActiveFrameColorForTest() const;
   SkColor GetInactiveFrameColorForTest() const;
 
+  views::Widget* frame() { return frame_; }
+
  protected:
   // Called when overview mode or split view state changed. If overview mode and
   // split view mode are both active at the same time, the header of the window

components/exo/shell_surface_base.cc

@@ -112,9 +112,16 @@ class CustomFrameView : public ash::CustomFrameViewAsh,
     SetVisible(enabled);
     if (!enabled)
       CustomFrameViewAsh::SetShouldPaintHeader(false);
+
+    frame()->GetNativeWindow()->AddObserver(this);
   }
 
-  ~CustomFrameView() override {}
+  ~CustomFrameView() override {
+    if (frame() && frame()->GetNativeWindow() &&
+        frame()->GetNativeWindow()->HasObserver(this)) {
+      frame()->GetNativeWindow()->RemoveObserver(this);
+    }
+  }
 
   // Overridden from ash::CustomFrameViewAsh:
   void SetShouldPaintHeader(bool paint) override {
@@ -156,9 +163,15 @@ class CustomFrameView : public ash::CustomFrameViewAsh,
     // the window mask layer bounds can be set correctly in function
     // SetShouldPaintHeader(). Note: this can be removed if the layer mask in
     // CustomFrameView becomes unnecessary.
+    // TODO(oshima): Investigate if we can eliminate this.
     CustomFrameViewAsh::UpdateHeaderView();
   }
 
+  void OnWindowDestroying(aura::Window* window) override {
+    DCHECK_EQ(frame()->GetNativeWindow(), window);
+    window->RemoveObserver(this);
+  }
+
   // Overridden from views::NonClientFrameView:
   gfx::Rect GetBoundsForClientView() const override {
     if (visible())