Fix compile error for gn build of mini_installer

Invalid conversion from size_t to DWORD on line 529.

BUG=82385
(not sure that bug is relevant but https://codereview.chromium.org/1580673004/
 made me think it was...)

Review URL: https://codereview.chromium.org/1662683002

Cr-Commit-Position: refs/heads/master@{#373261}
1 file changed