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This CL adds a cr-customize-themes element to
chrome://settings/manageProfile page behind a feature flag. This CL
focuses on the native side and it doesn't include necessary style
changes which will be included in future CLs. When the UI is ready, this
element will be put under the "NewProfilePicker" feature.

In order to use a cr-customize-themes component in WebUI, a
WebUIController class must enable mojo bindings and implement
customize_themes::mojom::CustomizeThemesHandlerFactory interface.
This CL applies these necessary changes to SettingsUI.

Screenshot with the feature flag enabled:
https://drive.google.com/file/d/1W1NITMHGbzHRzq1LeClp96Mh98B2MNGv/view?usp=sharing

Bug: 1129186
Change-Id: I18ead20f57e3bb0486e4821a946ea4e0aff3a242
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2414301
Commit-Queue: Monica Basta <msalama@chromium.org>
Reviewed-by: Mustafa Emre Acer <meacer@chromium.org>
Reviewed-by: dpapad <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809749}