In a non-incognito window, on my Win 10 box:

Omnibox dropdown, background: #ffffff
Omnibox dropdown, selected item: #f2f2f2 (contrast ratio 1.11)
Omnibox dropdown, hovered item: #eeeeee (contrast ratio 1.16)
Omnibox dropdown, hovered + selected item: #e0e0e0 (contrast ratio 1.32)
Chrome menu, background: #ffffff
Chrome menu, hovered/selected item: #ebebeb (contrast ratio 1.19)

There are several problems here:
* All these contrast levels are very low -- they're hard for me to see with good vision, and they wash out entirely in the presence of screen glare.  External users have complained.
* The omnibox and menus are similar but not identical.  We probably want them identical.  However, the concept of "selection vs. hover" is maybe immaterial in menus, so perhaps there's only a "selected" case there.
* Mouse hover is less important than selection (witness which one we use to set the text in the omnibox), but has higher contrast.

For reference, Windows' default selection color for these sorts of cases is #0078d7 (contrast ratio 4.49).  We also use this selection color in the omnibox for selected text, so having the dropdown not match is unusual.

Possible solutions:

(1) Previous versions of Chrome:
* Use the system selection color for selected items, whether or not they're hovered
* Use the system selection color with 0.5 alpha for hovered, non-selected items

(2) Google greys:
* Use grey 600 for hover (contrast ratio 3.68), grey 700 for selection (contrast ratio 6.04)

(3) Auto-computed contrast:
* For selection, use the background color, blended toward either grey 900 or the system selection color until hitting a contrast ratio 0f 4.5.  (Would result in something like a "grey 650" if using grey).
* For hover, do something similar but pick a lower number.