This turned out to be more complicated than expected.

Marking WontFix.

From upstream discussion it seems that there is an agreement on dropping the clamping altogether?

Yup, SGTM.

Upstream discussion here:
https://github.com/w3c/longtasks/issues/29

Is there a spec change required to go along with this?

Yes, I believe Shubhie is taking care of the spec bits?

Spec update in tiny, Nicolas you can just delete this line in the spec:
"also timing info provided is clamped to 1ms resolution" in Security section.

I've submitted pull requests upstream.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/23aed6337b63fc16197f2176e75c837655320a48

commit 23aed6337b63fc16197f2176e75c837655320a48
Author: Nicolas Pena <npm@chromium.org>
Date: Wed Aug 02 00:22:40 2017

Long tasks: do not clamp times

This CL drops clamping for long tasks to improve accuracy.
Discussion at https://github.com/w3c/longtasks/issues/29

Bug:  chromium:681718 
Change-Id: If6df344f0203f7b6347f54ffa49be8d6689b7d71
Reviewed-on: https://chromium-review.googlesource.com/587590
Reviewed-by: Shubhie Panicker <panicker@chromium.org>
Reviewed-by: Steve Kobes <skobes@chromium.org>
Commit-Queue: Nicolás Peña <npm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491166}
[modify] https://crrev.com/23aed6337b63fc16197f2176e75c837655320a48/third_party/WebKit/LayoutTests/external/wpt/longtask-timing/longtask-attributes.html
[modify] https://crrev.com/23aed6337b63fc16197f2176e75c837655320a48/third_party/WebKit/LayoutTests/external/wpt/longtask-timing/longtask-in-externalscript.html
[modify] https://crrev.com/23aed6337b63fc16197f2176e75c837655320a48/third_party/WebKit/LayoutTests/http/tests/performance-timing/longtask/longtask-attributes.html
[modify] https://crrev.com/23aed6337b63fc16197f2176e75c837655320a48/third_party/WebKit/LayoutTests/http/tests/performance-timing/longtask/longtask-externalscript.html
[modify] https://crrev.com/23aed6337b63fc16197f2176e75c837655320a48/third_party/WebKit/LayoutTests/http/tests/performance-timing/longtask/longtask-raf.html
[modify] https://crrev.com/23aed6337b63fc16197f2176e75c837655320a48/third_party/WebKit/Source/core/timing/PerformanceLongTaskTiming.cpp