cast, exo: Initialize display info list

Depending how early EXO is initialize the Screen object may have already
initialized its display state. When CastWMHelper adds its display
observer it will not get a OnDisplayAdded() event by default. Query the
list and initialize the observer to ensure state is correct.

Bug: b/138247323
Test: Manually on device to verify OnDisplayAdded() is called before EXO
clients call to ask for display info.

Change-Id: I473185fda846e64907eebb808141d51f281fbdde
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1720993
Auto-Submit: Daniel Nicoara <dnicoara@chromium.org>
Reviewed-by: Michael Spang <spang@chromium.org>
Commit-Queue: Daniel Nicoara <dnicoara@chromium.org>
Cr-Commit-Position: refs/heads/master@{#681384}

chromecast/browser/exo/cast_wm_helper.cc

@@ -53,6 +53,8 @@ CastWMHelper::CastWMHelper(
       cast_screen_(cast_screen),
       vsync_timing_manager_(this) {
   cast_screen_->AddObserver(&display_observer_);
+  for (const auto& display : cast_screen_->GetAllDisplays())
+    display_observer_.OnDisplayAdded(display);
 }
 
 CastWMHelper::~CastWMHelper() {