[Chromoting] Const accessor for displays

Change-Id: Icd07e1501d11914209048081496ca414d58a1f71
Reviewed-on: https://chromium-review.googlesource.com/c/1356023Reviewed-by: Joe Downing <joedow@chromium.org>
Commit-Queue: Gary Kacmarcik <garykac@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612433}

remoting/host/desktop_display_info.h

@@ -30,7 +30,7 @@ class DesktopDisplayInfo {
   bool operator==(const DesktopDisplayInfo& other);
   bool operator!=(const DesktopDisplayInfo& other);
 
-  std::vector<DisplayGeometry>& displays() { return displays_; };
+  const std::vector<DisplayGeometry>& displays() const { return displays_; };
 
  private:
   std::vector<DisplayGeometry> displays_;