Two concurrently running decoders will trigger data races on cpu_info_
which is lazily initialized on reading TestCpuFlag without proper
atomics.
BUG=libyuv:508
R=kjellander@webrtc.org
TEST=Running EndToEndTest.SendsAndReceivesMultipleStreams under TSan.
Review URL: https://codereview.webrtc.org/1414093003 .
Cr-Commit-Position: refs/heads/master@{#10335}