Fix system menu not appearing when clicking the corner of popup windows.

On Windows the system menu is expected to appear on a single left-click
to the top-left corner of the window, and a double click is expected to
close the window. The Views window icon added by custom titlebar was
intercepting the click and so the WM_NCLBUTTONDOWN message was not
getting DefWindowProc'ed. This patch fixes this by disabling event
handling on that icon, as it isn't supposed to do anything.

Bug: 828429
Change-Id: I1ee0cfe38707479948b51eed48da83639964a7ae
Reviewed-on: https://chromium-review.googlesource.com/1011544Reviewed-by: Peter Boström <pbos@chromium.org>
Commit-Queue: Bret Sepulveda <bsep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550472}

chrome/browser/ui/views/frame/glass_browser_frame_view.cc

@@ -109,6 +109,10 @@ GlassBrowserFrameView::GlassBrowserFrameView(BrowserFrame* frame,
     window_icon_ = new TabIconView(this, nullptr);
     window_icon_->set_is_light(true);
     window_icon_->set_id(VIEW_ID_WINDOW_ICON);
+    // Stop the icon from intercepting clicks intended for the HTSYSMENU region
+    // of the window. Even though it does nothing on click, it will still
+    // prevent us from giving the event back to Windows to handle properly.
+    window_icon_->set_can_process_events_within_subtree(false);
     AddChildView(window_icon_);
   }