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Mostly consolidate common pattern of declaring a pointer to the ChromeOS
browser policy connector into a helper function to be called within the
WebUI code.

Simultaneously, support detection for Windows and Apple devices.

Change-Id: Ib92d6e3ae88680cba1ff119bfa00391550a75388
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2346936
Commit-Queue: Daniel Hosseinian <dhoss@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#796692}