[Chromecast] Turn CastContentWindow into an abstract interface.

chromecast/browser/android/BUILD.gn

Index: chromecast/browser/android/BUILD.gn
diff --git a/chromecast/browser/android/BUILD.gn b/chromecast/browser/android/BUILD.gn
index 9de1c595d6dafb96da570eae19ad14e8ada127c2..fcf8b87ad3e72d9d67a1de7bb4d732c7a7558b5c 100644
--- a/chromecast/browser/android/BUILD.gn
+++ b/chromecast/browser/android/BUILD.gn
@@ -26,13 +26,12 @@ android_library("cast_shell_java") {
     "$java_src_dir/org/chromium/chromecast/shell/CastApplication.java",
     "$java_src_dir/org/chromium/chromecast/shell/CastAudioManager.java",
     "$java_src_dir/org/chromium/chromecast/shell/CastBrowserHelper.java",
+    "$java_src_dir/org/chromium/chromecast/shell/CastContentWindowAndroid.java",
     "$java_src_dir/org/chromium/chromecast/shell/CastCrashHandler.java",
     "$java_src_dir/org/chromium/chromecast/shell/CastCrashUploader.java",
     "$java_src_dir/org/chromium/chromecast/shell/CastMetricsHelper.java",
-    "$java_src_dir/org/chromium/chromecast/shell/CastShellActivity.java",
     "$java_src_dir/org/chromium/chromecast/shell/CastSysInfoAndroid.java",
-    "$java_src_dir/org/chromium/chromecast/shell/CastWindowAndroid.java",
-    "$java_src_dir/org/chromium/chromecast/shell/CastWindowManager.java",
+    "$java_src_dir/org/chromium/chromecast/shell/CastWebContentsActivity.java",
   ]
 
   android_manifest = cast_shell_android_manifest