WebRtcDataChannelTest.DataChannelGC is very flaky.

https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=content_browsertests&tests=WebRtcDataChannelTest.DataChannelGC

It fails about 1 in 4 builds on many bots, including Linux, Mac, Win, ChromeOS and Android.

Many are caught by automatic retries (the gray "TIMEOUT PASS" entries on the flakiness dashboard), but it still turns the build red sometimes, for example builds 2900 and 2912 of https://uberchromegw.corp.google.com/i/internal.client.clank/builders/asan-clang-phone?numbuilds=200.

The log output for those UNKNOWN failures was as follows:

C 6937.983s Main  [UNKNOWN] WebRtcDataChannelTest.DataChannelGC:
C 6937.983s Main  [ RUN      ] WebRtcDataChannelTest.DataChannelGC
C 6937.983s Main  [WARNING:dns_config_service_posix.cc(326)] Failed to read DnsConfig.
C 6937.983s Main  [ERROR:devtools_http_handler.cc(226)] Cannot start http server for devtools. Stop devtools.
C 6937.983s Main  [ERROR:child_process_launcher_helper_posix.cc(118)] Ignoring invalid file assets/snapshot_blob_64.bin
C 6937.983s Main  [WARNING:simple_synchronous_entry.cc(1102)] Could not open platform files for entry.
C 6937.983s Main  [ERROR:shell_android.cc(72)] Not implemented reached in void content::Shell::PlatformSetTitle(const base::string16 &): RTCDataChannel Content Browser Test
C 6937.983s Main  [INFO:CONSOLE(53)] "creating offer", source: http://127.0.0.1:56269/media/datachannel_test.html (53)
C 6937.983s Main  [INFO:CONSOLE(57)] "creating answer", source: http://127.0.0.1:56269/media/datachannel_test.html (57)
C 6937.983s Main  [INFO:CONSOLE(26)] "dc1/open event", source: http://127.0.0.1:56269/media/datachannel_test.html (26)
C 6937.983s Main  [INFO:CONSOLE(45)] "pc2/datachannel event", source: http://127.0.0.1:56269/media/datachannel_test.html (45)
C 6937.983s Main  [INFO:CONSOLE(48)] "dc2/message event: "message #1"", source: http://127.0.0.1:56269/media/datachannel_test.html (48)
C 6937.983s Main  [INFO:CONSOLE(48)] "dc2/message event: "message #2"", source: http://127.0.0.1:56269/media/datachannel_test.html (48)
C 6937.983s Main  [INFO:CONSOLE(48)] "dc2/message event: "message #3"", source: http://127.0.0.1:56269/media/datachannel_test.html (48)
C 6937.983s Main  [INFO:CONSOLE(48)] "dc2/message event: "message #4"", source: http://127.0.0.1:56269/media/datachannel_test.html (48)
C 6937.983s Main  [INFO:CONSOLE(48)] "dc2/message event: "message #5"", source: http://127.0.0.1:56269/media/datachannel_test.html (48)
C 6937.983s Main  [INFO:CONSOLE(48)] "dc2/message event: "message #6"", source: http://127.0.0.1:56269/media/datachannel_test.html (48)
C 6937.983s Main  [INFO:CONSOLE(48)] "dc2/message event: "message #7"", source: http://127.0.0.1:56269/media/datachannel_test.html (48)