[ios] Add HttpsUpgradeType enum for tracking upgraded HTTPS navigations

This CL adds a new enum to ios/web/public/navigation to keep track of
HTTPS upgrade reasons. Presently, all of the HTTPS upgrades on iOS
originate from the HTTPS-Only Mode feature. However, we'll soon
implement omnibox upgrades on iOS as well. This change allows us to
distinguish between HTTPS-Only Mode and Omnibox HTTPS upgrades.

See the design doc at https://docs.google.com/document/d/1YfO5wCR-nC8-hxB_o3Ic--Cxcur3foTJWH2pxp363Dk/edit for details.

Bug: 1195440
Change-Id: I3c3f778cdc95f19b98bffe7c5c1b013e830da900
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3715454
Reviewed-by: Ali Juma <ajuma@chromium.org>
Commit-Queue: Mustafa Emre Acer <meacer@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1016820}