Guidance from Shimi:

Mac and iOS: Title case for titles, buttons, menus, and menu items. Sentence case for other labels, like checkbox or radio button labels.

Chrome OS, Win, and Linux: Sentence case for everything, per Google UX writing guidelines

cc ramine@: FYI Mac and iOS are getting Title Case strings, which will make some of the payments UI untranslated for several weeks after the next branch.

Android should be using sentence case, by the way.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/273b2f6a3c4112d2dab0ecbcdc441eb48494f5cd

commit 273b2f6a3c4112d2dab0ecbcdc441eb48494f5cd
Author: Tommy Martino <tmartino@chromium.org>
Date: Tue Aug 15 20:58:13 2017

[WebPayments] Correcting string casing per-platform

This CL introduces Title-Cased strings to be used on iOS/Mac, and
ensures that sentence-casing is used on other platforms. Also lightly
reorganizes the existing strings, and removes a couple duplicate
or unused strings.

Bug:  753384 
Change-Id: Ibe1370736f95048fa29122e70c4c51b780c32033
Reviewed-on: https://chromium-review.googlesource.com/611659
Reviewed-by: Rouslan Solomakhin <rouslan@chromium.org>
Reviewed-by: Anthony Vallee-Dubois <anthonyvd@chromium.org>
Commit-Queue: Tommy Martino <tmartino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#494536}
[modify] https://crrev.com/273b2f6a3c4112d2dab0ecbcdc441eb48494f5cd/chrome/browser/ui/views/payments/payment_sheet_view_controller.cc
[modify] https://crrev.com/273b2f6a3c4112d2dab0ecbcdc441eb48494f5cd/chrome/browser/ui/views/payments/profile_list_view_controller.cc
[modify] https://crrev.com/273b2f6a3c4112d2dab0ecbcdc441eb48494f5cd/components/payments_strings.grdp