Add warning against changing strings in WebRtcEventLogManager

Add a comment warning against changing WebRtcEventLogManager-
related strings which may be relied upon by web-apps.

Bug: 775415
Change-Id: I7717d87437c72756ed2a2ecda61d9abb30a6a778
Reviewed-on: https://chromium-review.googlesource.com/1212943Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Commit-Queue: Elad Alon <eladalon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589486}

chrome/browser/media/webrtc/webrtc_event_log_manager_common.cc

@@ -42,6 +42,7 @@ static_assert(kInvalidWebRtcEventLogWebAppId < kMinWebRtcEventLogWebAppId ||
 
 const char kRemoteBoundWebRtcEventLogFileNamePrefix[] = "webrtc_event_log";
 
+// Important! These values may be relied on by web-apps. Do not change.
 const char kStartRemoteLoggingFailureFeatureDisabled[] = "Feature disabled.";
 const char kStartRemoteLoggingFailureUnlimitedSizeDisallowed[] =
     "Unlimited size disallowed.";