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The profile picker may be shown on new browsing sessions. This includes
Chrome being reopened from the dock.

In order to share code, two functions are moved out of
startup_browser_creator:
* IsGuestModeRequested
* ProfilePicker::ShouldShowAtLaunch
This is a simple code move and should not change the behavior. The only
behavior change is in app_controller_mac.mm

Tests for IsGuestModeRequested were added in this CL.

Fixed: 1122536
Change-Id: Ic078c79f300cde7ad5e26141eaebd3b249a66c4c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2485168Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Leonard Grey <lgrey@chromium.org>
Reviewed-by: Jan Krcal <jkrcal@chromium.org>
Commit-Queue: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825216}