[chromecast] Add FLAG_ACTIVITY_TASK_ON_HOME to CWCA.

This ensures that leaving the Cast Activity by pressing Back on
the remote will return to the Home screen, rather than whatever
app had been in the foreground previously.

Bug: Internal b/163998933
Test: launch Netflix with remote, Cast YouTube, press Back
Change-Id: Ib8936e874a80441a4e5c163794ff2576ffbef717
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2438991
Commit-Queue: Simeon Anfinrud <sanfin@chromium.org>
Commit-Queue: Luke Halliwell (slow) <halliwell@chromium.org>
Auto-Submit: Simeon Anfinrud <sanfin@chromium.org>
Reviewed-by: Luke Halliwell (slow) <halliwell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811895}

chromecast/browser/android/apk/src/org/chromium/chromecast/shell/CastWebContentsIntentUtils.java

@@ -330,7 +330,7 @@ public static Intent requestStartCastActivity(Context context, WebContents webCo
         intent.putExtra(INTENT_EXTRA_TURN_ON_SCREEN, turnOnScreen);
         intent.putExtra(INTENT_EXTRA_REMOTE_CONTROL_MODE, isRemoteControlMode);
         intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_SINGLE_TOP
-                | Intent.FLAG_ACTIVITY_NO_ANIMATION);
+                | Intent.FLAG_ACTIVITY_NO_ANIMATION | Intent.FLAG_ACTIVITY_TASK_ON_HOME);
         return intent;
     }