Thanks for the report - I'll address this, probably by lightening the blue used here.

These are NativeId::kColorId_ResultsTable{Normal,Hovered,Selected}Url so this ought not to be too hard to fix.

Colors locally:

background = 585B5E (Grey 800 blended with 14% white)
text = 2F7FDB (tricky to tell the actual color because of AA) but it's meant to be Blue Dark 600, which is 2581DF.

The paper contrast ratio of Blue Dark 600 vs Grey 800, ignoring AA, is 1.8 - even worse than you said :(

If I use Blue 300 (8AB4F8) for this case instead, I get 3.6:1, which passes at 18 regular and 14 bold, but not at 14 regular. If I put the background color up to Grey 900 in high contrast mode, I get 3F4043 for the background, which is at 5.2:1 and passes for all text sizes.

Conclusion:

In high contrast I will set the URL color for the selection to Blue 300 and the base background color to Grey 900.

https://chromium-review.googlesource.com/c/chromium/src/+/1163731

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/1d5e2accca9df7d4a886478c273716442aad3c64

commit 1d5e2accca9df7d4a886478c273716442aad3c64
Author: Elly Fong-Jones <ellyjones@chromium.org>
Date: Tue Aug 07 12:33:51 2018

omnibox: tweak high contrast scheme for URL readability

The URL's contrast on the selected grey background is too low. This change
increases the brightness of the URL and darkens the background, pushing the
contrast ratio over threshold.

Bug:  870290 
Change-Id: I30f40bcf85b283ee1ca6ddf82dd20e3dbeb9d91f
Reviewed-on: https://chromium-review.googlesource.com/1163731
Reviewed-by: Tommy Li <tommycli@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581203}
[modify] https://crrev.com/1d5e2accca9df7d4a886478c273716442aad3c64/chrome/browser/ui/omnibox/omnibox_theme.cc

URL contrast is now fixed in trunk. I won't merge this to M69.