Happening in browser_tests on multiple platform. Cause is similar to 
 issue 883680  that MessageLoopCurrent::Get()->IsIdleForTesting() is flaky.

../../chrome/browser/devtools/device/usb/android_usb_browsertest.cc:726: Failure
Value of: base::MessageLoopCurrent::Get()->IsIdleForTesting()
  Actual: false
Expected: true
Stack trace:
#0 0x563741fc68dc testing::internal::UnitTestImpl::CurrentOsStackTraceExceptTop()
#1 0x563741fc62b9 testing::internal::AssertHelper::operator=()
#2 0x563740762122 AndroidUsbCountTest_TestNoMultipleCallsRemoveAddInCallback_Test::RunTestOnMainThread()
#3 0x56374472dfcd content::BrowserTestBase::ProxyRunTestOnMainThreadLoop()
#4 0x5637442ec590 ChromeBrowserMainParts::PreMainMessageLoopRunImpl()
#5 0x5637442eb572 ChromeBrowserMainParts::PreMainMessageLoopRun()
#6 0x563741b25852 chromeos::ChromeBrowserMainPartsChromeos::PreMainMessageLoopRun()
#7 0x563742b94932 content::BrowserMainLoop::PreMainMessageLoopRun()
#8 0x563742f1f0e7 content::StartupTaskRunner::RunAllTasksNow()
#9 0x563742b9372d content::BrowserMainLoop::CreateStartupTasks()
#10 0x563742b96775 content::BrowserMainRunnerImpl::Initialize()
#11 0x563742b91c32 content::BrowserMain()
#12 0x563743e33b2f content::ContentMainRunnerImpl::RunServiceManager()
#13 0x563743e337b9 content::ContentMainRunnerImpl::Run()
#14 0x563745f3c833 service_manager::Main()
#15 0x563743e31e44 content::ContentMain()
#16 0x56374472dc01 content::BrowserTestBase::SetUp()
#17 0x56374429e144 InProcessBrowserTest::SetUp()


Fails for chromeos single process mash browser tests:
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/linux-chromeos-rel/17177
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/linux-chromeos-rel/17178

Fail pass on Linux, Mac, Win
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Linux%20Tests%20%28dbg%29%281%29/76373
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Mac10.11%20Tests/31663
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Win10%20Tests%20x64/29921