Add in-memory caching to PpdCache available printers.

chromeos/printing/ppd_cache.h

Index: chromeos/printing/ppd_cache.h
diff --git a/chromeos/printing/ppd_cache.h b/chromeos/printing/ppd_cache.h
index f18cc2064d979c8c1142b907537048024bc062c4..d44eeb5372d1dcb0301b85d3f45986f81fbd1176 100644
--- a/chromeos/printing/ppd_cache.h
+++ b/chromeos/printing/ppd_cache.h
@@ -73,13 +73,17 @@ class CHROMEOS_EXPORT PpdCache {
       const std::string& ppd_contents) = 0;
 
   // Return a map of available printers, if we have one available and it's
-  // not too stale.
-  virtual base::Optional<PpdProvider::AvailablePrintersMap>
-  FindAvailablePrinters() = 0;
+  // not too stale.  Returns null if no map is available.
+  //
+  // If a map is returned, ownership is retained by the cache.  The map is
+  // guaranteed to remain valid and unchanged until the next
+  // {Find|Store}AvailablePrinters call.
+  virtual const PpdProvider::AvailablePrintersMap* FindAvailablePrinters() = 0;
 
   // Store |available_printers|, replacing any existing entry.
   virtual void StoreAvailablePrinters(
-      const PpdProvider::AvailablePrintersMap& available_printers) = 0;
+      std::unique_ptr<PpdProvider::AvailablePrintersMap>
+          available_printers) = 0;
 };
 
 }  // namespace printing