That log output is in the "Steps" stdio log. e.g. https://build.chromium.org/p/tryserver.blink/builders/win10_blink_rel/builds/262/steps/steps/logs/stdio

Aye, in that log, it retries this test multiple times, each time it gets something like that:

11:52:56.820 5476 worker/6 compositing/squashing/squashing-print.html started
11:52:56.868 5476 worker/6 compositing/squashing/squashing-print.html output stderr lines:
11:52:56.868 5476   [5744:4440:0819/115256:5941218:ERROR:bitmap_platform_device_win.cc(138)] MapViewOfFile failed
11:52:56.868 5476   [5744:4440:0819/115256:5941218:ERROR:pixel_dump.cc(102)] Failed to create canvas width=800 height=600
11:52:56.885 4872 [7376/40813] compositing/squashing/squashing-print.html failed unexpectedly (image diff)
11:52:56.880 5476 worker/6 compositing/squashing/squashing-print.html failed:
11:52:56.880 5476 worker/6  image diff

It just so happens that I have a Win10 machine now; I'll try to run it there and make sure I see the same thing locally, and then see if I can debug or at least get more debugging clues.

Note, haven't gotten around to looking at this, and don't expect to within the next week; marking as P-3, but still planning to look into this if I have time later.

Update; still not planning to work on this very soon but I have had some progress getting Windows 10 machine set up to run layout tests to try to reproduce.

No expectations for this, so presumably not an issue any longer.