[Chromecast] Handle null in WebContentsRegistry.getWebContents.

If a session ID is not in the map, we propagate the null value to the
caller, which already handles null WebContents.

Bug: b/158602213
Test: cast_shell_junit_tests
Change-Id: Ib8d4d6fcaea7cf7a3df833199a379afe342b3aee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2238516
Reviewed-by: Simeon Anfinrud <sanfin@chromium.org>
Commit-Queue: Thoren Paulson <thoren@chromium.org>
Cr-Commit-Position: refs/heads/master@{#777515}

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

@@ -39,7 +39,8 @@ public class WebContentsRegistry {
     }
 
     public static WebContents getWebContents(String sessionId) {
-        return sSesionIdToWebContents.get(sessionId).webContents;
+        WebContentsHolder result = sSesionIdToWebContents.get(sessionId);
+        return result == null ? null : result.webContents;
     }
 
     public static void initializeWebContents(