Force rasterize the new layer for frame

This is required because when the virtual desks takes screenshot,
old layer is already detached, so it'll be copyed as a black.

Bug: 1083951
Test: maually testsed. See bug for repro step.

Change-Id: Ia26398d2d88557a70a970748623f665ac065c46d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2501970Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821510}

ash/public/cpp/frame_header.cc

@@ -135,8 +135,9 @@ class FrameHeader::FrameAnimatorView : public views::View,
 
     AddLayerBeneathView(old_layer);
 
-    // The old layer is on top and should fade out.
-    old_layer->SetOpacity(1.f);
+    // The old layer is on top and should fade out. Make it non opaque
+    // so that new layer gets resterized immediately.
+    old_layer->SetOpacity(0.99f);
     new_layer->SetOpacity(1.f);
     {
       ui::ScopedLayerAnimationSettings settings(old_layer->GetAnimator());