Check the current immersive state before setting the property

Updating to the same value can cause shelf state update. Check
the value before setting.

This is to merge 71. I'll change the impl side for 72.

Bug: 895744
Test: manual. see bug for repro step.
Change-Id: I9aad35098fc120dbfae365ddb79faf6af0069881
Reviewed-on: https://chromium-review.googlesource.com/c/1316590Reviewed-by: Tao Wu <wutao@chromium.org>
Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605119}

ash/public/cpp/immersive/immersive_fullscreen_controller.cc

@@ -297,7 +297,9 @@ void ImmersiveFullscreenController::UnlockRevealedState() {
 // static
 void ImmersiveFullscreenController::EnableForWidget(views::Widget* widget,
                                                     bool enabled) {
-  widget->GetNativeWindow()->SetProperty(kImmersiveIsActive, enabled);
+  auto* window = widget->GetNativeWindow();
+  if (window->GetProperty(kImmersiveIsActive) != enabled)
+    widget->GetNativeWindow()->SetProperty(kImmersiveIsActive, enabled);
 }
 
 ////////////////////////////////////////////////////////////////////////////////