Add @Nullable annotations to quiet errorprone.

Those are preventive annotations to prepare for incoming android update
(coming with Chromium roll).
Currently the roll is blocked partly because errorprone complains!

Bug: webrtc:11095, chromium:1003532
Change-Id: If4e2879a522e895ce7fb1f2a9ad36d06f98f2a61
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/160002
Reviewed-by: Sami Kalliomäki <sakal@webrtc.org>
Commit-Queue: Yves Gerey <yvesg@google.com>
Cr-Commit-Position: refs/heads/master@{#29830}

sdk/android/src/java/org/webrtc/WebRtcClassLoader.java

@@ -18,6 +18,10 @@ package org.webrtc;
 class WebRtcClassLoader {
   @CalledByNative
   static Object getClassLoader() {
-    return WebRtcClassLoader.class.getClassLoader();
+    Object loader = WebRtcClassLoader.class.getClassLoader();
+    if (loader == null) {
+      throw new RuntimeException("Failed to get WebRTC class loader.");
+    }
+    return loader;
   }
 }