assigning to atwilson@ since https://chromium-review.googlesource.com/719838 might be relevant.  thanks!

in all three tests, the failure looks roughly like this:

[ RUN      ] VideoPlayerBrowserTest.CheckInitialElements
[23877:23877:0213/111415.763975:WARNING:user_policy_manager_factory_chromeos.cc(184)] No policy loaded for known non-enterprise user
[23877:23877:0213/111415.838258:WARNING:user_session_manager.cc(1044)] Attempting to save user password for non enterprise user.
[23877:23877:0213/111416.686212:ERROR:network_type_pattern.cc(134)] NetworkTypePattern: wifi: Can not match empty type.
[23877:23877:0213/111416.687820:ERROR:network_type_pattern.cc(134)] NetworkTypePattern: wifi: Can not match empty type.
[23877:23877:0213/111416.690661:ERROR:network_type_pattern.cc(134)] NetworkTypePattern: wifi: Can not match empty type.
[23877:23877:0213/111416.691218:ERROR:network_type_pattern.cc(134)] NetworkTypePattern: wifi: Can not match empty type.
[23877:23877:0213/111417.991685:ERROR:wallpaper_controller.cc(1247)] User is ephemeral or guest! Fallback to default wallpaper.
[23877:23877:0213/111422.379325:INFO:CONSOLE(4988)] "Requesting volume list.", source: chrome-extension://hhaomjibdihmijegdhdafkllkbggdgoj/background/js/background_common_scripts.js (4988)
[23877:23877:0213/111423.207402:INFO:CONSOLE(4990)] "Volume list fetched with: 2 items.", source: chrome-extension://hhaomjibdihmijegdhdafkllkbggdgoj/background/js/background_common_scripts.js (4990)
[23877:23877:0213/111423.208069:INFO:CONSOLE(5000)] "Initializing volume: downloads:Downloads", source: chrome-extension://hhaomjibdihmijegdhdafkllkbggdgoj/background/js/background_common_scripts.js (5000)
[23877:23877:0213/111423.208779:INFO:CONSOLE(5403)] "Requesting file system.", source: chrome-extension://hhaomjibdihmijegdhdafkllkbggdgoj/background/js/background_common_scripts.js (5403)
[23877:23877:0213/111423.243439:INFO:CONSOLE(5000)] "Initializing volume: drive:drive-user", source: chrome-extension://hhaomjibdihmijegdhdafkllkbggdgoj/background/js/background_common_scripts.js (5000)
[23877:23877:0213/111423.244148:INFO:CONSOLE(5403)] "Requesting file system.", source: chrome-extension://hhaomjibdihmijegdhdafkllkbggdgoj/background/js/background_common_scripts.js (5403)
[23877:23877:0213/111423.303119:INFO:CONSOLE(1164)] "Cache database creating or upgrading.", source: chrome-extension://pmfjbimdmchhbnneeidfognadeopoehp/background_scripts.js (1164)
[23877:23877:0213/111423.517602:FATAL:private_api_file_system.cc(809)] Check failed: external_backend->CanHandleType(file_system_url.type()). 
#0 0x7f72c346761d base::debug::StackTrace::StackTrace()
#1 0x7f72c3465c4c base::debug::StackTrace::StackTrace()
#2 0x7f72c34e78cd logging::LogMessage::~LogMessage()
#3 0x000003716791 extensions::FileManagerPrivateInternalResolveIsolatedEntriesFunction::RunAsync()
#4 0x000008a38a9d ChromeAsyncExtensionFunction::Run()
#5 0x000003eb8650 ExtensionFunction::RunWithValidation()
#6 0x000003ebefdf extensions::ExtensionFunctionDispatcher::DispatchWithCallbackInternal()
#7 0x000003ebe01e extensions::ExtensionFunctionDispatcher::Dispatch()
#8 0x000003f27b2c extensions::ExtensionWebContentsObserver::OnRequest()
#9 0x000003e3bc72 _ZN3IPC20DispatchToMethodImplIN10extensions21AppWindowContentsImplEMS2_FvPN7content15RenderFrameHostERKNSt3__16vectorINS1_15DraggableRegionENS6_9allocatorIS8_EEEEES4_NS6_5tupleIJSB_EEEJLm0EEEEvPT_T0_PT1_OT2_NS6_16integer_sequenceImJXspT3_EEEE
#10 0x000003e3bba0 _ZN3IPC16DispatchToMethodIN10extensions21AppWindowContentsImplEN7content15RenderFrameHostEJRKNSt3__16vectorINS1_15DraggableRegionENS5_9allocatorIS7_EEEEENS5_5tupleIJSA_EEEEENS5_9enable_ifIXeqsZT1_sr3std10tuple_sizeINS5_5decayIT2_E4typeEEE5valueEvE4typeEPT_MSM_FvPT0_DpT1_ESP_OSH_
#11 0x000003f281b1 _ZN3IPC8MessageTI29ExtensionHostMsg_Request_MetaNSt3__15tupleIJ31ExtensionHostMsg_Request_ParamsEEEvE8DispatchIN10extensions28ExtensionWebContentsObserverES9_N7content15RenderFrameHostEMS9_FvPSB_RKS4_EEEbPKNS_7MessageEPT_PT0_PT1_T2_
#12 0x000003f27a9c extensions::ExtensionWebContentsObserver::OnMessageReceived()
#13 0x000008a39aea extensions::ChromeExtensionWebContentsObserver::OnMessageReceived()
#14 0x7f72bb9cc8d3 content::WebContentsImpl::OnMessageReceived()
#15 0x7f72bb04071c content::RenderFrameHostImpl::OnMessageReceived()

No, doesn't look like my change - that's a purely informative debug message stating that the user is known not to be policy controlled so we aren't initializing the policy framework (so none of the code in cl/719838 is being exercised).

I think it's worth looking into why that private filesystem api is being invoked - +hirono as an OWNER of that code.