JNI wrapper for PeerConnection::SetBitrate.

BUG=webrtc:7395

Review-Url: https://codereview.webrtc.org/2868413004
Cr-Commit-Position: refs/heads/master@{#19243}

webrtc/sdk/android/instrumentationtests/src/org/webrtc/PeerConnectionTest.java

@@ -815,6 +815,10 @@ public class PeerConnectionTest {
     answeringExpectations.dataChannel.close();
     offeringExpectations.dataChannel.close();
 
+    // Test SetBitrate.
+    assertTrue(offeringPC.setBitrate(100000, 5000000, 500000000));
+    assertFalse(offeringPC.setBitrate(3, 2, 1));
+
     // Free the Java-land objects and collect them.
     shutdownPC(offeringPC, offeringExpectations);
     offeringPC = null;