Add an API to get total size of Drive cache including non-evictable ones.

components/drive/chromeos/file_cache.h

Index: components/drive/chromeos/file_cache.h
diff --git a/components/drive/chromeos/file_cache.h b/components/drive/chromeos/file_cache.h
index 2d17202e0db0935486550a142acd804bc1d86aaf..ab23b62719373fdb8f3552053382c9056b7e0346 100644
--- a/components/drive/chromeos/file_cache.h
+++ b/components/drive/chromeos/file_cache.h
@@ -93,8 +93,11 @@ class FileCache {
   // false.
   bool FreeDiskSpaceIfNeededFor(int64_t num_bytes);
 
+  // Calculates and returns cache size. In error case, this returns 0.
+  int64_t CalculateCacheSize();
+
   // Calculates and returns evictable cache size. In error case, this returns 0.
-  uint64_t CalculateEvictableCacheSize();
+  int64_t CalculateEvictableCacheSize();
 
   // Checks if file corresponding to |id| exists in cache, and returns
   // FILE_ERROR_OK with |cache_file_path| storing the path to the file.