Somewhere between 64.0.3251.0 (good) and 64.0.3252.0 (bad), our jstests.arc_welder tests started failing.

https://logs.chromium.org/v/?s=chrome%2Fbb%2Fclient.arc%2Fnacl-x86_64-lkgr%2F7233%2F%2B%2Frecipes%2Fsteps%2Fintegration_tests_nacl_x86_64%2F0%2Fstdout

3486:3486:1026/212800.167751:INFO:CONSOLE(2)] "INFO: [ RUN      ] ArcWelderTest.GetMetadataFromUIHasValidDefaultMetadata", source: chrome-extension://emfinbmielocnlhgmfkkmkngdoccbadn/gen_test.min.js (2)
[3486:3518:1026/212800.240877:ERROR:service_manager.cc(157)] Connection InterfaceProviderSpec prevented service: content_renderer from binding interface: blink::mojom::ReportingServiceProxy exposed by: content_browser
[3486:3486:1026/212800.241068:INFO:CONSOLE(0)] "/deep/ combinator is no longer supported in CSS dynamic profile.It is now effectively no-op, acting as if it were a descendant combinator. /deep/ combinator will be removed, and will be invalid at M65. You should remove it. See https://www.chromestatus.com/features/4964279606312960 for more details.", source:  (0)
[3486:3486:1026/212800.257675:INFO:CONSOLE(17096)] "Uncaught InvalidStateError: Failed to execute 'createShadowRoot' on 'Element': Shadow root cannot be created on a host which already hosts a shadow tree.", source: chrome-extension://emfinbmielocnlhgmfkkmkngdoccbadn/index.js (17096)
[3486:3486:1026/212800.262584:INFO:CONSOLE(17096)] "Uncaught InvalidStateError: Failed to execute 'createShadowRoot' on 'Element': Shadow root cannot be created on a host which already hosts a shadow tree.", source: chrome-extension://emfinbmielocnlhgmfkkmkngdoccbadn/index.js (17096)
[3486:3486:1026/212800.262838:INFO:CONSOLE(17096)] "Uncaught InvalidStateError: Failed to execute 'createShadowRoot' on 'Element': Shadow root cannot be created on a host which already hosts a shadow tree.", source: chrome-extension://emfinbmielocnlhgmfkkmkngdoccbadn/index.js (17096)
[3486:3486:1026/212800.263199:INFO:CONSOLE(17096)] "Uncaught InvalidStateError: Failed to execute 'createShadowRoot' on 'Element': Shadow root cannot be created on a host which already hosts a shadow tree.", source: chrome-extension://emfinbmielocnlhgmfkkmkngdoccbadn/index.js (17096)
[3486:3486:1026/212800.271905:INFO:CONSOLE(17096)] "Uncaught InvalidStateError: Failed to execute 'createShadowRoot' on 'Element': Shadow root cannot be created on a host which already hosts a shadow tree.", source: chrome-extension://emfinbmielocnlhgmfkkmkngdoccbadn/index.js (17096)
[3486:3486:1026/212800.351847:INFO:CONSOLE(2)] "ERROR: TypeError: Cannot read property 'dataset' of null
    at getMetadataFromUI (chrome-extension://emfinbmielocnlhgmfkkmkngdoccbadn/gen_main.min.js:91:197)
    at $jscomp.scope.ArcWelderTest.<anonymous> (chrome-extension://emfinbmielocnlhgmfkkmkngdoccbadn/gen_test.min.js:62:531)
    at chrome-extension://emfinbmielocnlhgmfkkmkngdoccbadn/gen_test.min.js:13:383
    at <anonymous>", source: chrome-extension://emfinbmielocnlhgmfkkmkngdoccbadn/gen_test.min.js (2)
[3486:3486:1026/212800.353513:INFO:CONSOLE(2)] "INFO: [  FAILED  ] ArcWelderTest.GetMetadataFromUIHasValidDefaultMetadata (184ms)", source: chrome-extension://emfinbmielocnlhgmfkkmkngdoccbadn/gen_test.min.js (2)