Add More Base Path Helpers to DeviceFiles

(This is a merge of http://go/wvgerrit/13910/ from the Widevine CDM
repository.)

DeviceFiles has a lot of repeated code whenever it needs to check for
file existence or remove a file. When reading and writing files, it
has wrappers that handle this repeated burden. This change adds
wrappers for the other functionality used by DeviceFiles as well, to
reduce duplication.

Change-Id: If959b504672c1b907346d28f31648d8028de8bdf

libwvdrmengine/cdm/core/include/device_files.h

@@ -3,6 +3,8 @@
 #ifndef WVCDM_CORE_DEVICE_FILES_H_
 #define WVCDM_CORE_DEVICE_FILES_H_
 
+#include <unistd.h>
+
 #include "scoped_ptr.h"
 #include "wv_cdm_types.h"
 
@@ -81,9 +83,14 @@ class DeviceFiles {
                                  CdmKeyResponse* license_response);
 
  private:
+  // Helpers that wrap the File interface and automatically handle hashing, as
+  // well as adding the device files base path to to the file name.
   bool StoreFileWithHash(const char* name, const std::string& serialized_file);
   bool StoreFileRaw(const char* name, const std::string& serialized_file);
   bool RetrieveHashedFile(const char* name, std::string* serialized_file);
+  bool FileExists(const char* name);
+  bool RemoveFile(const char* name);
+  ssize_t GetFileSize(const char* name);
 
   // Certificate and offline licenses are now stored in security
   // level specific directories. In an earlier version they were