The following two tests started to timeout with an average frequency of 33% (1 timeout each 3 runs) around r278005 (I'm not implying r278005 caused this, but it looks that around this commit it started to happen the flakiness regarding timeouts for this two tests).
On GTK bot the issue seems more easily reproducible.
This two tests are also reported as flaky timeouts on Mac. See bug 223759 and bug 224842
Updated expectations on r279545