DiceBrowserTest.Signin failed several times during the last few days and caused (at least) these bot failures:

https://uberchromegw.corp.google.com/i/chromium.win/builders/Win7%20Tests%20%281%29/builds/69047
https://uberchromegw.corp.google.com/i/chromium.win/builders/Win7%20Tests%20%281%29/builds/69056
https://uberchromegw.corp.google.com/i/chromium.win/builders/Win7%20Tests%20%281%29/builds/69083
https://uberchromegw.corp.google.com/i/chromium.win/builders/Win7%20Tests%20%281%29/builds/69118

So please have a look at the failures in chrome/browser/signin/dice_browsertest.cc:230
and check out 
https://uberchromegw.corp.google.com/i/chromium.win/builders/Win7%20Tests%20%281%29?numbuilds=100
for the latest ones.

If you cannot find a short-term solution, consider disabling  this test.