Keep CastWebContentsActivity alive in background

When an Android activity moves into the background, we want to keep the
cast app alive. The app should stay alive until onDestroy.

CQ-DEPEND=CL:756932
TEST=Cast YT, start another app, kill app to bring Cast to foreground
BUG=b:68987926

Change-Id: I0235401acdf88a0f497f183493a6467e2b178e7a
Reviewed-on: https://chromium-review.googlesource.com/757663
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Commit-Queue: Aidan Wolter <awolter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#515770}

diff --git a/chromecast/chromecast.gni b/chromecast/chromecast.gni
index f77a3bd..de07990 100644
--- a/chromecast/chromecast.gni
+++ b/chromecast/chromecast.gni

@@ -50,6 +50,10 @@
 
   # Set to true to start music apps in headless mode.
   enable_headless_music_mode = false
+
+  # Set to true to keep Android activities alive when they are moved to the
+  # background.
+  enable_background_activities = false
 }
 
 declare_args() {