[Creation flow] Add a native back button to sign-in

This CL adds a native view toolbar on top of the Google sign-in page. So
far, this only contains a back button. A location bar will be added
in a follow-up CL.

The CL also extends the WidgetDelegate to be able to specify
ThemeProvider for the widget. This is needed because the profile picker
window exists without any Browser window as its parent widget and
native elements are used (and more of them will be used) that expect
existence of a ThemeProvider.

Bug: 1126913
Change-Id: I95286052338757619589bfbf1112acd06bd07fc1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2569752
Commit-Queue: Jan Krcal <jkrcal@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841874}