Reland "Test oobe screens for public session login"

This is a reland of 144e9e93

Making user session manager open a browser on startup fixed this
for me locally (without it, chrome shutdown would start just after
login screen UI goes away).

To be safe added a waiter to make sure the accept button is there when
the test attempts clicking it.

TBR=alemate@chromium.org

Original change's description:
> Test oobe screens for public session login
>
> Tests that the terms of service screen (if required by policy) is the
> only screen shown to public session user during login.
>
> To make environment setup for different configurations eaiser, separate
> end to end test setup steps from OobeInteractiveUiTest into a separate
> mixin.
>
> BUG=none
>
> Change-Id: Ib6580e2b4f63f59c7b262fb0f79982197251724c
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1607103
> Reviewed-by: Alexander Alekseev <alemate@chromium.org>
> Commit-Queue: Toni Baržić <tbarzic@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#660269}

Bug: none
Change-Id: Ib3e9a7adf3486122ae3ddb5c8c2cd4a8e87915ca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1614151Reviewed-by: Toni Baržić <tbarzic@chromium.org>
Commit-Queue: Toni Baržić <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#660656}

ash/login/ui/arrow_button_view.cc

@@ -5,6 +5,7 @@
 #include "ash/login/ui/arrow_button_view.h"
 
 #include "ash/resources/vector_icons/vector_icons.h"
+#include "ui/accessibility/ax_node_data.h"
 #include "ui/gfx/canvas.h"
 #include "ui/gfx/paint_vector_icon.h"
 
@@ -45,6 +46,11 @@ void ArrowButtonView::PaintButtonContents(gfx::Canvas* canvas) {
   // Draw arrow icon.
   views::ImageButton::PaintButtonContents(canvas);
 }
+void ArrowButtonView::GetAccessibleNodeData(ui::AXNodeData* node_data) {
+  // TODO(tbarzic): Fix this - https://crbug.com/961930.
+  if (GetAccessibleName().empty())
+    node_data->SetNameExplicitlyEmpty();
+}
 
 void ArrowButtonView::SetBackgroundColor(SkColor color) {
   background_color_ = color;

ash/login/ui/arrow_button_view.h

@@ -20,6 +20,7 @@ class ArrowButtonView : public LoginButton {
 
   // views::Button:
   void PaintButtonContents(gfx::Canvas* canvas) override;
+  void GetAccessibleNodeData(ui::AXNodeData* node_data) override;
 
   // Set background color of the button.
   void SetBackgroundColor(SkColor color);