Raise threshold for UKM vs JS value comparison in LCP browser test

It seems the values can be off by a bit more than 1 ms, likely due to
differences in the way the offset from 'navigation start' is computed
for the UKM values vs for the JS values and due to coarsening of the
values in the JS API.

Example where the delta is greater than 1:
https://ci.chromium.org/ui/p/chromium/builders/try/win10_chromium_x64_rel_ng/902221/test-results

The flakes on Linux seem mostly due to timeouts in the script, hence not
fixed by this CL.

Bug: 1223602
Change-Id: I984a3c02426e2f619414c843890e3a064f1083c0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3002226
Reviewed-by: Annie Sullivan <sullivan@chromium.org>
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#898181}