[iOS] Implement Content Analysis Info for Enterprise

Refactor and make InitializeRequest to be pure virtual method in the
base class and make both desktop and iOS implement it.

Implement and populate fields that are meaningful to iOS in our version
of Content Analysis Info.

Bug: 485126116
Change-Id: Ib009fab0f1a9a6c3de63e195dcdbded1bc7ff8bc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7684830
Reviewed-by: Ernesto Izquierdo Clua <eic@google.com>
Reviewed-by: Nancy Xiao <nancylanxiao@google.com>
Reviewed-by: Xu Li <mxlg@google.com>
Commit-Queue: Xu Li <mxlg@google.com>
Cr-Commit-Position: refs/heads/main@{#1604200}