Fail DCHECK if 0/ERROR_SUCCESS is passed into OSErrorToFileError.

The prior two values are not considered to be errors, and will be
mapped to FILE_ERROR_FAILED. Adding the DCHECK to detect incorrect
use of this function.

Bug: None
Change-Id: Id8dcfa01bf18010fc1a59fc20defb907475aac6c
Reviewed-on: https://chromium-review.googlesource.com/798050
Commit-Queue: Chris Mumford <cmumford@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521234}

base/files/file_posix.cc

@@ -424,6 +424,8 @@ File::Error File::OSErrorToFileError(int saved_errno) {
       UMA_HISTOGRAM_SPARSE_SLOWLY("PlatformFile.UnknownErrors.Posix",
                                   saved_errno);
 #endif
+      // This function should only be called for errors.
+      DCHECK_NE(0, saved_errno);
       return FILE_ERROR_FAILED;
   }
 }

base/files/file_win.cc

@@ -312,6 +312,8 @@ File::Error File::OSErrorToFileError(DWORD last_error) {
     default:
       UMA_HISTOGRAM_SPARSE_SLOWLY("PlatformFile.UnknownErrors.Windows",
                                   last_error);
+      // This function should only be called for errors.
+      DCHECK_NE(static_cast<DWORD>(ERROR_SUCCESS), last_error);
       return FILE_ERROR_FAILED;
   }
 }