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..7d6408f5efe93e47e18e676415c406b5cd9727f6 100644
--- a/base/platform_file.h
+++ b/base/platform_file.h
@@ -204,6 +204,12 @@ BASE_EXPORT bool TouchPlatformFile(PlatformFile file,
 // Returns some information for the given file.
 BASE_EXPORT bool GetPlatformFileInfo(PlatformFile file, PlatformFileInfo* info);
 
+#if defined(OS_WIN)
+PlatformFileError LastErrorToPlatformFileError(DWORD saved_errno);
+#elif defined(OS_POSIX)
+PlatformFileError ErrnoToPlatformFileError(int saved_errno);
+#endif
+
 // Use this class to pass ownership of a PlatformFile to a receiver that may or
 // may not want to accept it.  This class does not own the storage for the
 // PlatformFile.