Relax threading requirements in VideoEncoder.

Previous strict threading requirements we not always followed and this
causes crashes in some scenarios.

Bug: webrtc:7760
Change-Id: I4808edfff19c63a9c4a9585834d46f9c73bb1c4b
Reviewed-on: https://webrtc-review.googlesource.com/13660
Reviewed-by: Oleh Prypin <oprypin@webrtc.org>
Commit-Queue: Oleh Prypin <oprypin@webrtc.org>
Commit-Queue: Sami Kalliomäki <sakal@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20353}

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

@@ -68,6 +68,9 @@ public interface VideoDecoder {
    * infinite number of frames before the decoded frame is consumed.
    */
   boolean getPrefersLateDecoding();
-  /** Should return a descriptive name for the implementation. */
+  /**
+   * Should return a descriptive name for the implementation. Gets called once and cached. May be
+   * called from arbitrary thread.
+   */
   String getImplementationName();
 }