Reset |most_recent_bounds_| on new Overlay

When overlays are enabled, on Nexus 5 devices, upon exiting and
re-entering fullscreen, nothing is displayed on screen.

This is caused by the fact that AVDACodecImage never resets
|most_recent_bounds_| when a new overlay is received. This means
AndroidOverlay::ScheduleLayout() is never called in
AVDACodecImage::ScheduleOverlayPlane(), and new overlays retains their
initial bounds of [0,0,1,1].

This CL fixes the issue by reseting the bounds when a new shared state
(and therefore potential new Overlay) is set.

BUG=728802
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2916193002
Cr-Commit-Position: refs/heads/master@{#476524}

media/gpu/avda_codec_image.cc

@@ -148,6 +148,7 @@ bool AVDACodecImage::SetSharedState(
   if (shared_state == shared_state_)
     return false;
   shared_state_ = shared_state;
+  most_recent_bounds_ = gfx::Rect();
   return true;
 }