Fix msan flake in rtcstats_integrationtest.cc.
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-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#16187}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 2d4d653e1fd25ade9ae768058b1225e6b430bc76

Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk