Update Java MediaStream when native stream's set of tracks changes.

This will handle the scenario where, for example, the initial
offer/answer only negotiates audio, and video is added later (to the
same stream). Previously, there was absolutely no way to get a handle to
the new track without hacking the SDP. Now, the stream will be updated
after setRemoteDescription finishes.

Bug: webrtc:5677
Change-Id: Iea31bb7744da6b82afdaf44c8f74d721298a9474
Reviewed-on: https://webrtc-review.googlesource.com/6261
Reviewed-by: Sami Kalliomäki <sakal@webrtc.org>
Commit-Queue: Taylor Brandstetter <deadbeef@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20228}

sdk/android/src/jni/jni_helpers.h

@@ -184,6 +184,10 @@ class Iterable {
     // Advances the iterator one step.
     Iterator& operator++();
 
+    // Removes the element the iterator is pointing to. Must still advance the
+    // iterator afterwards.
+    void Remove();
+
     // Provides a way to compare the iterator with itself and with the end
     // iterator.
     // Note: all other comparison results are undefined, just like for C++ input
@@ -200,6 +204,7 @@ class Iterable {
     jobject value_ = nullptr;
     jmethodID has_next_id_ = nullptr;
     jmethodID next_id_ = nullptr;
+    jmethodID remove_id_ = nullptr;
     rtc::ThreadChecker thread_checker_;
 
     RTC_DISALLOW_COPY_AND_ASSIGN(Iterator);