Make base:ReplaceFile return an informative error.

base/platform_file.h

Index: base/platform_file.h
diff --git a/base/platform_file.h b/base/platform_file.h
index 43758c51cee035771fe267ff8b7b57cd773cd828..6a6b9434fe58980624f1f2b39f7b1f9a896503e5 100644
--- a/base/platform_file.h
+++ b/base/platform_file.h
@@ -19,14 +19,6 @@
 
 namespace base {
 
-#if defined(OS_WIN)
-typedef HANDLE PlatformFile;
-const PlatformFile kInvalidPlatformFileValue = INVALID_HANDLE_VALUE;
-#elif defined(OS_POSIX)
-typedef int PlatformFile;
-const PlatformFile kInvalidPlatformFileValue = -1;
-#endif
-
 // PLATFORM_FILE_(OPEN|CREATE).* are mutually exclusive. You should specify
 // exactly one of the five (possibly combining with other flags) when opening
 // or creating a file.
@@ -119,6 +111,16 @@ struct BASE_EXPORT PlatformFileInfo {
   base::Time creation_time;
 };
 
+#if defined(OS_WIN)
+typedef HANDLE PlatformFile;
+const PlatformFile kInvalidPlatformFileValue = INVALID_HANDLE_VALUE;
+PlatformFileError LastErrorToPlatformFileError(DWORD saved_errno);
+#elif defined(OS_POSIX)
+typedef int PlatformFile;
+const PlatformFile kInvalidPlatformFileValue = -1;
+PlatformFileError ErrnoToPlatformFileError(int saved_errno);
+#endif
+
 // Creates or opens the given file. If |created| is provided, it will be set to
 // true if a new file was created [or an old one truncated to zero length to
 // simulate a new file, which can happen with PLATFORM_FILE_CREATE_ALWAYS], and