dpranke@ - could you PTAL and help find a good owner for this?

Just out of curious: if we don't specify that a layout test is slow, what are the consequences? The test could pass or fail randomly?

Without the [ Slow ] expectation, the test would flakily time out more often. 
 Marking a test as [ Slow ] increases the timeout - this means that a test is less likely to hit the timeout due to random variations in the test runtime.

From third_party/WebKit/LayoutTests/SlowTests:

# This file should only mark tests as Slow. If they are failing in some other way, that listing should be in TestExpectations.
# Lines listed in both files will have the *union* of the expectations.
#
# In general, we should list tests in this file if they are slower than ~30% of the default timeout time, as we
# can see a 3x variance or more in test times on the bots. In practice, this means that any test that takes longer
# than 2 seconds in Release mode or 6 seconds in Debug mode should be listed here.
#
# Further documentation:
# https://chromium.googlesource.com/chromium/src/+/master/docs/testing/layout_test_expectations.md

@ned - feel free to either take this or triage further?

It's unlikely for me to get to this bug, so unassign myself