Should this apply to the current "Email Page Location" link too or leave that alone? (To put it another way: replace current string or add new string?)

Is there a different location that string appears, other than in the share command?

It's the menu option that the Share Menu's replacing.

Will they ever appear simultaneously? Does this string appear on other platforms? I need a bit more. 

I think it should be updated but need to understand what I'm asking for.

No they wouldn't appear simultaneously (just want to dot i's and cross t's here since the option will still be available for a bit and the string change might precede shipping share menu).

It's a Mac-only string.

Ok! In that case, I don't think we should change the old string. Let's introduce the new string with the new feature.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/875cf13e107f44d264b24b579fba5abeb51d4c99

commit 875cf13e107f44d264b24b579fba5abeb51d4c99
Author: Leonard Grey <lgrey@chromium.org>
Date: Wed Nov 29 21:20:38 2017

[Mac] Change share email share extension menu string

Per discussion on bug, adding a new string instead of changing the old
one.

Bug:  788016 
Change-Id: I91a5f941fe6e91a04cdac08cd6c14aea6244e033
Reviewed-on: https://chromium-review.googlesource.com/797442
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Leonard Grey <lgrey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520247}
[modify] https://crrev.com/875cf13e107f44d264b24b579fba5abeb51d4c99/chrome/app/generated_resources.grd
[modify] https://crrev.com/875cf13e107f44d264b24b579fba5abeb51d4c99/chrome/browser/ui/cocoa/share_menu_controller.mm