Tests:

TabindexOpenDialog_FileManagerBrowserTest.Test_0
TabindexSaveFileDialog_FileManagerBrowserTest.Test

Flakes observed on the  linux_chromium_chromeos_rel_ng trybot. Seemingly unrelated CLs show the same tests failing:

https://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_chromeos_rel_ng/builds/218357
https://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_chromeos_rel_ng/builds/218787
https://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_chromeos_rel_ng/builds/218312


Sample trace:

---------
[3773:3773:0526/004916:WARNING:merge_session_throttling_utils.cc(134)] Loading content for a profile without session restore?
[3773:3773:0526/004916:WARNING:merge_session_throttling_utils.cc(134)] Loading content for a profile without session restore?
[3773:3773:0526/004916:WARNING:merge_session_throttling_utils.cc(134)] Loading content for a profile without session restore?
[3773:3773:0526/004917:WARNING:merge_session_throttling_utils.cc(134)] Loading content for a profile without session restore?
[3773:3773:0526/004917:INFO:CONSOLE(112)] "Element #filename-input-textbox:focus (maybe in iframe undefined) is not found.", source: chrome-extension://oobinhbdbiehknkpbpejbbpdbkdjmoco/test_util.js (112)
[3773:3773:0526/004917:WARNING:merge_session_throttling_utils.cc(134)] Loading content for a profile without session restore?
[3773:3773:0526/004917:WARNING:merge_session_throttling_utils.cc(134)] Loading content for a profile without session restore?
BrowserTestBase signal handler received SIGTERM. Backtrace:
#0 0x000001a63ade base::debug::StackTrace::StackTrace()
#1 0x000001b19a3a content::(anonymous namespace)::DumpStackTraceSignalHandler()
#2 0x7f3d6d3280b0 <unknown>
#3 0x7f3d6d3d9993 __poll
#4 0x7f3d7156cff6 <unknown>
#5 0x7f3d7156d124 g_main_context_iteration
#6 0x000001ae0696 base::MessagePumpGlib::Run()
#7 0x000001a7e84d base::MessageLoop::RunHandler()
#8 0x000001a9b6a0 base::RunLoop::Run()
#9 0x00000606b96c content::RunThisRunLoop()
#10 0x00000606b90a content::RunMessageLoop()
#11 0x000001258777 file_manager::FileManagerBrowserTestBase::RunTestMessageLoop()
#12 0x0000012584c6 file_manager::FileManagerBrowserTestBase::StartTest()
#13 0x000001a535cb InProcessBrowserTest::RunTestOnMainThreadLoop()
#14 0x000001b19814 content::BrowserTestBase::ProxyRunTestOnMainThreadLoop()
#15 0x0000063c42db ChromeBrowserMainParts::PreMainMessageLoopRunImpl()
#16 0x0000063c335e ChromeBrowserMainParts::PreMainMessageLoopRun()
#17 0x000002059811 chromeos::ChromeBrowserMainPartsChromeos::PreMainMessageLoopRun()
#18 0x000002e85fcd content::BrowserMainLoop::PreMainMessageLoopRun()
#19 0x0000030198b7 content::StartupTaskRunner::RunAllTasksNow()
#20 0x000002e847c8 content::BrowserMainLoop::CreateStartupTasks()
#21 0x000003102d02 content::BrowserMainRunnerImpl::Initialize()
#22 0x000006bf8b66 content::BrowserMain()
#23 0x00000604b17e content::ContentMainRunnerImpl::Run()
#24 0x000006049b60 content::ContentMain()
#25 0x000001b191c8 content::BrowserTestBase::SetUp()
#26 0x000001a51e46 InProcessBrowserTest::SetUp()
#27 0x000001e50398 testing::Test::Run()
#28 0x000001e5130a testing::TestInfo::Run()
#29 0x000001e51793 testing::TestCase::Run()
#30 0x000001e58989 testing::internal::UnitTestImpl::RunAllTests()
#31 0x000001e5862e testing::UnitTest::Run()
#32 0x000001af8206 base::TestSuite::Run()
#33 0x000001a0addb ChromeTestSuiteRunner::RunTestSuite()
#34 0x00000606876e content::LaunchTests()
#35 0x0000011766c7 main
#36 0x7f3d6d3137ed __libc_start_main
#37 0x0000005c9719 <unknown>
------------


Assigned to ryoh@ according to the test's log, and cc'ed the current sheriffs.