The E2E two_client_extensions_sync_test cases have been flaky on Mac bots. 

The flaky tests are:
TwoClientExtensionsSyncTest.Add_E2ETest
TwoClientExtensionsSyncTest.Uninstall_E2ETest
TwoClientExtensionsSyncTest.UpdateEnableDisableExtension_E2ETest
TwoClientExtensionsSyncTest.DISABLED_StartWithDifferentExtensions_E2ETest

Unfortunately the logs do not show much. It seems that the tests times out but it is not clear:
https://uberchromegw.corp.google.com/i/internal.client.kitchensync/builders/Mac%20Dev/builds/447/steps/sync_integration_tests%20on%20Mac-10.9/logs/TwoClientExtensionsSyncTest.Add_E2ETest

...

[4808:1287:0323/090443:WARNING:resource_bundle_mac.mm(133)] ResourceBundle::GetNativeImageNamed() is returning a cached gfx::Image that isn't backed by an NSImage. The image will be converted, rather than going through the NSImage loader. resource_id = 7415
[4808:1287:0323/090443:WARNING:shell.cc(302)] Instance: exe:chrome attempting to register an instance for a process it created for target: exe:chrome_renderer without the mojo:shell{client_process} capability class.
[4820:1287:0323/090443:WARNING:histogram_base.cc(131)] 2 histograms were created before reporting was enabled.
[4808:1287:0323/090443:WARNING:resource_bundle_mac.mm(133)] ResourceBundle::GetNativeImageNamed() is returning a cached gfx::Image that isn't backed by an NSImage. The image will be converted, rather than going through the NSImage loader. resource_id = 7079
[4808:1287:0323/090444:INFO:CONSOLE(9)] "SW registered", source: https://www.google.com/_/chrome/newtab?espv=2&ie=UTF-8 (9)
[4808:1287:0323/090445:WARNING:sync_extension_helper.cc(299)] Unable to convert extension name "Web Store" to index
[4808:1287:0323/090445:WARNING:sync_extension_helper.cc(299)] Unable to convert extension name "Bookmark Manager" to index
[4808:1287:0323/090445:WARNING:sync_extension_helper.cc(299)] Unable to convert extension name "Cloud Print" to index
[4808:1287:0323/090445:WARNING:sync_extension_helper.cc(299)] Unable to convert extension name "Chromium" to index
[4808:1287:0323/090445:WARNING:sync_extension_helper.cc(299)] Unable to convert extension name "Chromium PDF Viewer" to index
[4808:60679:0323/090445:WARNING:mac_util.mm(272)] Failed to set backup exclusion for file '/private/var/folders/9x/6c6sv3cj4j53wzpzthbp4ksm0000gm/T/.org.chromium.Chromium.CQbOb1/dWaAE6Z/.org.chromium.Chromium.LpuJHg/History': Error Domain=NSOSStatusErrorDomain Code=-50 "The operation couldn?t be completed. (OSStatus error -50.)" (paramErr: error in user parameter list) (-50)
[4808:1287:0323/090445:WARNING:resource_bundle_mac.mm(133)] ResourceBundle::GetNativeImageNamed() is returning a cached gfx::Image that isn't backed by an NSImage. The image will be converted, rather than going through the NSImage loader. resource_id = 7415