Return texture frame when dropping frames in CameraCapturer.

BUG=webrtc:6411,b/31686979

Review-Url: https://codereview.webrtc.org/2363043002
Cr-Commit-Position: refs/heads/master@{#14365}

webrtc/api/android/java/src/org/webrtc/CameraCapturer.java

@@ -158,6 +158,7 @@ public abstract class CameraCapturer implements CameraVideoCapturer {
       synchronized (stateLock) {
         if (session != currentSession) {
           Logging.w(TAG, "onTextureFrameCaptured from another session.");
+          surfaceHelper.returnTextureFrame();
           return;
         }
         if (!firstFrameObserved) {