Not sure when this started, as the code has not changed in several weeks. It's been happening for at least a couple days now.

glider: Did you see this one on your shift?

e.g. https://build.chromium.org/p/chromium.memory.fyi/builders/Linux%20Tests%20%28valgrind%29%285%29/builds/40942

Leak_DefinitelyLost
292 (40 direct, 252 indirect) bytes in 1 blocks are definitely lost in loss record 40,758 of 41,552
  operator new(unsigned long) (m_replacemalloc/vg_replace_malloc.c:1140)
  _ZN4base4BindIMNS_12_GLOBAL__N_121PostTaskAndReplyRelayEFvvEJNS_8internal17UnretainedWrapperIS2_EEEEENS_8CallbackINS5_22MakeUnboundRunTypeImplIT_JDpT0_EE4TypeELNS5_8CopyModeE1EEESA_DpOSB_ (base/bind.h:106)
  base::internal::PostTaskAndReplyImpl::PostTaskAndReply(tracked_objects::Location const&, base::Callback<void (), (base::internal::CopyMode)1> const&, base::Callback<void (), (base::internal::CopyMode)1> const&) (base/threading/post_task_and_reply_impl.cc:83)
  base::TaskRunner::PostTaskAndReply(tracked_objects::Location const&, base::Callback<void (), (base::internal::CopyMode)1> const&, base::Callback<void (), (base::internal::CopyMode)1> const&) (/mnt/data/b/build/slave/chromium-rel-linux-valgrind-tests-5/build/src/out/Release/unit_tests)
  bool base::PostTaskAndReplyWithResult<net::WifiPHYLayerProtocol, net::WifiPHYLayerProtocol>(base::TaskRunner*, tracked_objects::Location const&, base::Callback<net::WifiPHYLayerProtocol (), (base::internal::CopyMode)1> const&, base::Callback<void (net::WifiPHYLayerProtocol), (base::internal::CopyMode)1> const&) (/mnt/data/b/build/slave/chromium-rel-linux-valgrind-tests-5/build/src/out/Release/unit_tests)
  metrics::NetworkMetricsProvider::ProbeWifiPHYLayerProtocol() (components/metrics/net/network_metrics_provider.cc:154)
  metrics::NetworkMetricsProvider::NetworkMetricsProvider(base::TaskRunner*) (components/metrics/net/network_metrics_provider.cc:38)
  ChromeMetricsServiceClient::Initialize() (chrome/browser/metrics/chrome_metrics_service_client.cc:372)
  ChromeMetricsServiceClient::Create(metrics::MetricsStateManager*, PrefService*) (chrome/browser/metrics/chrome_metrics_service_client.cc:205)
  ChromeMetricsServicesManagerClient::CreateMetricsServiceClient() (/mnt/data/b/build/slave/chromium-rel-linux-valgrind-tests-5/build/src/out/Release/unit_tests)
  metrics_services_manager::MetricsServicesManager::GetMetricsServiceClient() (components/metrics_services_manager/metrics_services_manager.cc:61)
  metrics_services_manager::MetricsServicesManager::GetMetricsService() (/mnt/data/b/build/slave/chromium-rel-linux-valgrind-tests-5/build/src/out/Release/unit_tests)
  BrowserProcessImpl::metrics_service() (/mnt/data/b/build/slave/chromium-rel-linux-valgrind-tests-5/build/src/out/Release/unit_tests)
  IOThread::IOThread(PrefService*, policy::PolicyService*, net_log::ChromeNetLog*, extensions::EventRouterForwarder*) (chrome/browser/io_thread.cc:509)
  BrowserProcessImpl::PreCreateThreads() (chrome/browser/browser_process_impl.cc:998)
  BrowserProcessImplTest_LifeCycle_Test::TestBody() (chrome/browser/browser_process_impl_unittest.cc:83)
Suppression (error hash=#1FCE8081F47A3D4D#):
  For more info on using suppressions see http://dev.chromium.org/developers/tree-sheriffs/sheriff-details-chromium/memory-sheriff#TOC-Suppressing-memory-reports
{
   <insert_a_suppression_name_here>
   Memcheck:Leak
   fun:_Znw*
   fun:_ZN4base4BindIMNS_12_GLOBAL__N_121PostTaskAndReplyRelayEFvvEJNS_8internal17UnretainedWrapperIS2_EEEEENS_8CallbackINS5_22MakeUnboundRunTypeImplIT_JDpT0_EE4TypeELNS5_8CopyModeE1EEESA_DpOSB_
   fun:_ZN4base8internal20PostTaskAndReplyImpl16PostTaskAndReplyERKN15tracked_objects8LocationERKNS_8CallbackIFvvELNS0_8CopyModeE1EEESB_
   fun:_ZN4base10TaskRunner16PostTaskAndReplyERKN15tracked_objects8LocationERKNS_8CallbackIFvvELNS_8internal8CopyModeE1EEESB_
   fun:_ZN4base26PostTaskAndReplyWithResultIN3net20WifiPHYLayerProtocolES2_EEbPNS_10TaskRunnerERKN15tracked_objects8LocationERKNS_8CallbackIFT_vELNS_8internal8CopyModeE1EEERKNS9_IFvT0_ELSD_1EEE
   fun:_ZN7metrics22NetworkMetricsProvider25ProbeWifiPHYLayerProtocolEv
   fun:_ZN7metrics22NetworkMetricsProviderC1EPN4base10TaskRunnerE
   fun:_ZN26ChromeMetricsServiceClient10InitializeEv
   fun:_ZN26ChromeMetricsServiceClient6CreateEPN7metrics19MetricsStateManagerEP11PrefService
   fun:_ZN34ChromeMetricsServicesManagerClient26CreateMetricsServiceClientEv
   fun:_ZN24metrics_services_manager22MetricsServicesManager23GetMetricsServiceClientEv
   fun:_ZN24metrics_services_manager22MetricsServicesManager17GetMetricsServiceEv
   fun:_ZN18BrowserProcessImpl15metrics_serviceEv
   fun:_ZN8IOThreadC1EP11PrefServicePN6policy13PolicyServiceEPN7net_log12ChromeNetLogEPN10extensions20EventRouterForwarderE
   fun:_ZN18BrowserProcessImpl16PreCreateThreadsEv
   fun:_ZN37BrowserProcessImplTest_LifeCycle_Test8TestBodyEv
}