Our mocks have used lots of font sizes for these. :-P
Is the intention exactly 15px, or is it matching a text size constant for Material design used throughout similar UI surfaces?

Patti, could you take this?

The intention is to do whatever the suite of chrome dialogs are doing under #secondary-ui-md :). My answer is vague because I can't confidently answer your question. bsep@ will have a better answer if need be. 

Yes, 15pt is the default CONTEXT_DIALOG_TITLE (there's code that specifically changes it to be "1 pixel smaller than the default" title size). For now I'm assuming all the stuff in the Harmony changes should only apply when --secondary-ui-md is on, though (except the accessibility changes, those seem good to have regardless of Harmony), which includes this change, so when I get around to doing this it'll be Harmony-only.

>> (there's code that specifically changes it to be "1 pixel smaller than the default" title size)

What's the purpose for this LOC? Unless im missing something, it doesn't seem necessary

RE #c5 - I'm not sure, I think you would have to ask Lucas (or possibly even whoever was working on this before Lucas). This was actually fixed in https://chromium-review.googlesource.com/c/chromium/src/+/711534 though (landed as r511350), mainly to reduce the amount of Harmony-specific code in the Page Info bubble. So the title text size is actually now always 15pt regardless of whether Harmony is on or not.