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This CL adds a checkbox to the main view of the profile picker. By
default the profile picker frequency is set to be shown on every
startup.  The user can decide to change that by checking/unchecking the
'Ask on startup' option. The profile picker is meant to be shown in the
picker window and is a singleton not in a chrome tab, for that reason
the profile picker does not listen on changes to
|kBrowserShowProfilePickerOnStartup|.

Screenshot:
https://drive.google.com/file/d/1UH63Pl773SQ95iSLzxnnaPTjwJ5KwxJD/view?usp=sharing

Bug: 1063856
Change-Id: I2e8f71abb8ad93a3e0f5dfd84426a7deb0ea15b1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2333834
Commit-Queue: Monica Basta <msalama@chromium.org>
Reviewed-by: Esmael Elmoslimany <aee@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#794525}