This test sometimes fails on Mac like so:

[ RUN      ] ContinueWhereILeftOffTest.SessionCookiesCloseAllBrowsers
2017-04-07 08:51:45.969 browser_tests[3340:46887] NSWindow warning: adding an unknown subview: <FullSizeContentView: 0x7fb123d8f3b0>
2017-04-07 08:51:45.969 browser_tests[3340:46887] Call stack:
(
    "+callStackSymbols disabled for performance reasons"
)
[08:51:46.096] vtDecompressionDuctCreate signalled err=-8973 (err) (Could not select and open decoder instance) at /SourceCache/CoreMedia_frameworks/CoreMedia-1562.240/Sources/VideoToolbox/VTDecompressionSession.c line 1181
<<<< VTVideoEncoderSelection >>>> VTSelectAndCreateVideoEncoderInstanceInternal: no video encoder found for 'avc1'

[08:51:46.232] VTSelectAndCreateVideoEncoderInstanceInternal signalled err=-12908 (err) (Video encoder not available) at /SourceCache/CoreMedia_frameworks/CoreMedia-1562.240/Sources/VideoToolbox/VTVideoEncoderSelection.c line 1245
[08:51:46.232] VTCompressionSessionCreate signalled err=-12908 (err) (Could not select and open encoder instance) at /SourceCache/CoreMedia_frameworks/CoreMedia-1562.240/Sources/VideoToolbox/VTCompressionSession.c line 946
[3340:779:0407/085146.708536:WARNING:ipc_message_attachment_set.cc(49)] MessageAttachmentSet destroyed with unconsumed attachments: 0/1
../../chrome/test/base/ui_test_utils.cc:494: Failure
Value of: chrome::GetTotalBrowserCount()
  Actual: 1
Expected: original_browsers_.size() + 1
Which is: 2
[  FAILED  ] ContinueWhereILeftOffTest.SessionCookiesCloseAllBrowsers, where TypeParam =  and GetParam() =  (1583 ms)

For example, see: https://uberchromegw.corp.google.com/i/chromium.mac/builders/Mac10.10%20Tests/builds/16272

thomasanderson@, can you take a look?