Hold a reference to AndroidVideoTrackSource while calling onFrameCaptured.

This makes it safe to deliver frames to the sink from VideoProcessor
even after setSink has been called with null reference without danger
of use after free.

Bug: b/148063550
Change-Id: Ib78f75ac49fc6117f744c55da1a4e671bbdcdf22
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/168160
Reviewed-by: Paulina Hensman <phensman@webrtc.org>
Commit-Queue: Sami Kalliomäki <sakal@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30455}

sdk/android/api/org/webrtc/MediaSource.java

@@ -25,9 +25,11 @@ public class MediaSource {
     }
   }
 
+  private final RefCountDelegate refCountDelegate;
   private long nativeSource;
 
   public MediaSource(long nativeSource) {
+    refCountDelegate = new RefCountDelegate(() -> JniCommon.nativeReleaseRef(nativeSource));
     this.nativeSource = nativeSource;
   }
 
@@ -38,7 +40,7 @@ public class MediaSource {
 
   public void dispose() {
     checkMediaSourceExists();
-    JniCommon.nativeReleaseRef(nativeSource);
+    refCountDelegate.release();
     nativeSource = 0;
   }
 
@@ -48,6 +50,20 @@ public class MediaSource {
     return nativeSource;
   }
 
+  /**
+   * Runs code in {@code runnable} holding a reference to the media source. If the object has
+   * already been released, does nothing.
+   */
+  void runWithReference(Runnable runnable) {
+    if (refCountDelegate.safeRetain()) {
+      try {
+        runnable.run();
+      } finally {
+        refCountDelegate.release();
+      }
+    }
+  }
+
   private void checkMediaSourceExists() {
     if (nativeSource == 0) {
       throw new IllegalStateException("MediaSource has been disposed.");