With instructions from tsmith, I found this using grizzly  to fuzz the Canvas 2D API.
The attached test case causes a 30+ second Flush  on my machine during a Snapshot for line:
Currently with remote canvas it will timeout after a number of waits and this can then cause the Translation to crash.
In a previous change I added a check to make sure the other side is still alive by checking the IPDL channel, so I'm just going to remove the limit on the number of waits.
This gives a similar behaviour to when this is just running in the content process, because that just has to wait on any long running Direct2D calls.
I'm setting this to block Release not Nightly, because I think these sorts of waits are probably fairly extreme examples.