ui-devtools: Add tracking of aura::Window visibility

Makes it easy to tell if the aura::Window is visible or not since this
information can't always be deduced from Layer or occlusion state.

BUG=None
TEST=Manually by inspecting an aura window that was hidden/visible.

Change-Id: I17f513de4b14f412ea6195e6ac2d928c5ba8e2ff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2283234
Commit-Queue: Daniel Nicoara <dnicoara@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#785800}

components/ui_devtools/views/window_element.cc

@@ -91,6 +91,8 @@ WindowElement::GetCustomPropertiesForMatchedStyle() const {
   // change OcclusionState::UNKNOWN to UNKNOWN
   state_str = state_str.substr(state_str.find("::") + 2);
   cur_properties.emplace_back("occlusion-state", state_str);
+  cur_properties.emplace_back("is-visible",
+                              window_->IsVisible() ? "true" : "false");
   cur_properties.emplace_back("surface",
                               window_->GetSurfaceId().is_valid()
                                   ? window_->GetSurfaceId().ToString()