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This CL adds the logic for requesting the desktop version of web sites
by default when kDefaultToDesktopOnIPad is enabled.
It also adds the logic for when to request desktop/mobile based on
the size classes.
The user agent types are used for three things:
1. Updating the user agent sent to the web page.
2. Storing the user agent used for back/forward navigation and
   knowing what request (mobile or desktop) to display in the menus.
3. Inheriting from one item to the other.

1 and 2 should be kept in sync. 3 can be different.
So this CL introduces a UserAgentType used for inheritance, which can
be different from the one used for updating the user agent.

Bug: 1025225
Change-Id: Iad219985ec2625979e2cc7f2aa0f1eb4dbfacec5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1919352
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Reviewed-by: Ali Juma <ajuma@chromium.org>
Cr-Commit-Position: refs/heads/master@{#721965}