Fix Views web-modal dialog widget creation.

Fix Views web-modal dialog widget creation.

r272016 regressed web-modal dialog shadow styling.
Use CreateDialogWidget for dialog creation to fix that.
Only set SHADOW_TYPE_NONE for new-style dialogs there.
(removes the double border, consolidates dialog widget init)
Take WidgetDelegate for ConstrainedWebDialogDelegateViewViews.
Mark web-modals as ui::MODAL_TYPE_CHILD and child widgets.

Remove now unused Widget::CreateWindowAsFramelessChild.
(keep a copy for the system-modal captive portal dialog)
(that dialog is very odd, system-modal and "web-modal"?)

Add [Create|Show]WebModalDialogViews helper functions.
Use these for c/b/ui/views web-modal dialog init/show.
(cannot be used by c/b/chromeos, so cleanup less there)
Cleanup includes, cached widgets, using statements, etc.

BUG=376646,378970
TEST=Views tab-modal dialogs appear as expected.
R=wittman@chromium.org,sky@chromium.org

Review URL: https://codereview.chromium.org/301393002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@274991 0039d316-1c4b-4281-b951-d872f2087c98