update_engine: Use utils::FileSize when finding the size of a file.

The utils::FileSize funciton can find the size of a block device correctly.
Use it instead of the adhoc methods used around the codebase.

BUG=chromium:415867
TEST=Ran a butterfly-paladin tryjob with --hwtest.

Change-Id: Id6fd37f04b136b4265bde9b1f56c379a5d9c30f9
Reviewed-on: https://chromium-review.googlesource.com/217418
Reviewed-by: Alex Deymo <deymo@chromium.org>
Commit-Queue: Gabe Black <gabeblack@chromium.org>
Tested-by: Gabe Black <gabeblack@chromium.org>
diff --git a/p2p_manager_unittest.cc b/p2p_manager_unittest.cc
index 56ba33b..456c8ad 100644
--- a/p2p_manager_unittest.cc
+++ b/p2p_manager_unittest.cc
@@ -208,19 +208,19 @@
 static bool CheckP2PFile(const string& p2p_dir, const string& file_name,
                          ssize_t expected_size, ssize_t expected_size_xattr) {
   string path = p2p_dir + "/" + file_name;
-  struct stat statbuf;
   char ea_value[64] = { 0 };
   ssize_t ea_size;
 
-  if (stat(path.c_str(), &statbuf) != 0) {
+  off_t p2p_size = utils::FileSize(path);
+  if (p2p_size < 0) {
     LOG(ERROR) << "File " << path << " does not exist";
     return false;
   }
 
   if (expected_size != 0) {
-    if (statbuf.st_size != expected_size) {
+    if (p2p_size != expected_size) {
       LOG(ERROR) << "Expected size " << expected_size
-                 << " but size was " << statbuf.st_size;
+                 << " but size was " << p2p_size;
       return false;
     }
   }