So, it looks like what's happening here is:
- The omnibox popup is being created at startup
- Each of the result cells sets " - " as text on its separator view
- The linked change caches OmniboxTextView preferred sizes when text is changed
... but I didn't realize the separator view was one, so the cached size is never used. This part might not be totally relevant to this issue, but I'll address it regardless

Normally I would say "maybe we don't want to construct the Omnibox popup at startup at all", but it looks like doing the text shaping *doubles* construction time. There's some stuff about creating glyph caches in the flame graph, so my theory is that.

Before getting someone who understands Windows text layout involved, I'm curious if 
https://chromium-review.googlesource.com/c/chromium/src/+/1123456 fixes the regression, since it will cause only 1 shape vs. one for each match cell.

Otherwise, I think we would need to weigh this vs. savings while typing in the Omnibox from not shaping every time when results aren't changing.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/6aac6ccf30deb69d888247a853b9087610606bd4

commit 6aac6ccf30deb69d888247a853b9087610606bd4
Author: Leonard Grey <lgrey@chromium.org>
Date: Mon Jul 09 14:50:17 2018

Views: Use cached size for Omnibox match separator

Missed that the separator was an OmniboxTextView.

Bug: 860789, 712268
Change-Id: I670fa25754f23a5aaaa9201f548d9b735f92ecf3
Reviewed-on: https://chromium-review.googlesource.com/1128187
Reviewed-by: Justin Donnelly <jdonnelly@chromium.org>
Commit-Queue: Leonard Grey <lgrey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#573304}
[modify] https://crrev.com/6aac6ccf30deb69d888247a853b9087610606bd4/chrome/browser/ui/views/omnibox/omnibox_match_cell_view.cc

wittman@ are we still seeing this as a regression or did it cancel out in layouts? I probably don't want to pull the change either way, but we might be able to split the difference somehow on creating the popup.

The regression is still there. The change above landed in 69.0.3487.0 and the diff between 69.0.3486.0 and 69.0.3487.0 shows no improvement: https://uma.googleplex.com/p/chrome/callstacks?sid=b726db3d43124a46971bbb314ca42ba1