Ash HUD: Add latest graph values to the legend.

This CL adds latest graph values to the legend to make current
system state easier to read.

Screenshots:
https://screenshot.googleplex.com/7GEoQ34yCo4LGuX.png
https://screenshot.googleplex.com/A2jgLi6Nt8x6erc.png

Bug: 1132183
Change-Id: I4d0ac763487fe3259da22cbe84e624ddd2ef9688
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2430090
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Alexander Alekseev <alemate@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811575}

diff --git a/ash/hud_display/graph_page_view_base.h b/ash/hud_display/graph_page_view_base.h
index 9e20393..de0060c 100644
--- a/ash/hud_display/graph_page_view_base.h
+++ b/ash/hud_display/graph_page_view_base.h

@@ -19,6 +19,7 @@
 namespace hud_display {
 
 class Grid;
+class Legend;
 
 // Interface for a single graph page.
 class GraphPageViewBase : public views::View, public views::ButtonListener {
@@ -49,6 +50,9 @@
                    int horizontal_points_number,
                    int horizontal_ticks_interval);
 
+ protected:
+  void RefreshLegendValues();
+
  private:
   // Container for the Grid object.
   views::View* grid_container_ = nullptr;  // not owned
@@ -56,7 +60,7 @@
   // Container for the legend object.
   views::View* legend_container_ = nullptr;              // not owned
   views::ImageButton* legend_min_max_button_ = nullptr;  // not owned
-  views::View* legend_ = nullptr;                        // not owned
+  Legend* legend_ = nullptr;                             // not owned
 
   SEQUENCE_CHECKER(ui_sequence_checker_);
 };