Introduce Nullable annotation.

Just adds the annotation to base_java for now to check that this does
not break any downstream targets.

Bug: webrtc:8881
Change-Id: I9425020e36be5e52447cec592a4474a9eb09b5bd
Reviewed-on: https://webrtc-review.googlesource.com/51960
Commit-Queue: Sami Kalliomäki <sakal@webrtc.org>
Reviewed-by: Patrik Höglund <phoglund@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22526}

webrtc.gni

@@ -496,8 +496,15 @@ if (is_android) {
                                "visibility",
                              ])
 
+      javac_args = []
+
       # Treat warnings as errors.
-      javac_args = [ "-Werror" ]
+      javac_args += [ "-Werror" ]
+
+      # Add any arguments defined by the invoker.
+      if (defined(invoker.javac_args)) {
+        javac_args += invoker.javac_args
+      }
 
       no_build_hooks = true
     }