Add test annotation to PeerConnectionClientTest.testLoopbackVp9 test.

Test annotations were added to all tests but
PeerConnectionClientTest.testLoopbackVp9 in
https://codereview.webrtc.org/1876233002.
Recent changes in the Chromium toolchain now makes
the test fail if annotations are missing.

TBR=phoglund@webrtc.org

Review URL: https://codereview.webrtc.org/1962533002 .

Cr-Commit-Position: refs/heads/master@{#12658}

webrtc/examples/androidtests/src/org/appspot/apprtc/test/PeerConnectionClientTest.java

@@ -370,7 +370,7 @@ public class PeerConnectionClientTest extends InstrumentationTestCase
     doLoopbackTest(createParametersForVideoCall(VIDEO_CODEC_VP8, false), false);
   }
 
-  //@SmallTest
+  @SmallTest
   public void testLoopbackVp9() throws InterruptedException {
     doLoopbackTest(createParametersForVideoCall(VIDEO_CODEC_VP9, false), false);
   }