WebRTC: Re-enable 30s datachannel telemetry test.

BUG=chromium:726811

Review-Url: https://codereview.chromium.org/2908383002
Cr-Commit-Position: refs/heads/master@{#475797}

tools/perf/page_sets/webrtc_cases.py

@@ -152,7 +152,3 @@ class WebrtcExpectations(story.expectations.StoryExpectations):
     self.DisableStory('audio_call_isac/1600_10s',
                       [story.expectations.ALL],
                       'crbug.com/468732')
-
-    self.DisableStory('30s_datachannel_transfer',
-                      [story.expectations.ALL_DESKTOP],
-                      'crbug.com/726811')