Fix msan flake in rtcstats_integrationtest.cc.

This CL https://codereview.webrtc.org/2641763003 changed echo return
loss /...enhancement stats from being optional to being undefined
because that was the observed behavior (and a TODO was added to
investigate why).

It turns out that these stats are sometimes available, e.g. if the test
runs for a while like MSAN bot does, so this turned the test flaky.
Example failure:
https://build.chromium.org/p/client.webrtc/builders/Linux%20MSan/builds/8242

This CL reverts that change without reverting the rest of the CL which
other CLs depend on, and updates the TODO.

BUG=chromium:627816
TBR=hta@webrtc.org
NOTRY=True
NOPRESUBMIT=True

Review-Url: https://codereview.webrtc.org/2640743007
Cr-Commit-Position: refs/heads/master@{#16187}
diff --git a/webrtc/api/rtcstats_integrationtest.cc b/webrtc/api/rtcstats_integrationtest.cc
index 5ded447..f89207a 100644
--- a/webrtc/api/rtcstats_integrationtest.cc
+++ b/webrtc/api/rtcstats_integrationtest.cc
@@ -471,13 +471,13 @@
       verifier.TestMemberIsUndefined(media_stream_track.full_frames_lost);
       // Audio-only members
       verifier.TestMemberIsNonNegative<double>(media_stream_track.audio_level);
-      // TODO(hbos): Find out why |echo_return_loss| and
-      // |echo_return_loss_enhancement| are undefined in the integration test.
-      // Is this a real problem or a test problem? Update test so that they are.
-      // crbug.com/627816
-      verifier.TestMemberIsUndefined(media_stream_track.echo_return_loss);
-      verifier.TestMemberIsUndefined(
-          media_stream_track.echo_return_loss_enhancement);
+      // TODO(hbos): |echo_return_loss| and |echo_return_loss_enhancement| are
+      // flaky on msan bot (sometimes defined, sometimes undefined). Should the
+      // test run until available or is there a way to have it always be
+      // defined? crbug.com/627816
+      verifier.MarkMemberTested(media_stream_track.echo_return_loss, true);
+      verifier.MarkMemberTested(
+          media_stream_track.echo_return_loss_enhancement, true);
     }
     return verifier.ExpectAllMembersSuccessfullyTested();
   }