Make Win runtime DLLs writeable before overwriting

This should fix the VS2015 bot which is failing because we can't
overwrite read-only DLL files.

BUG=skia:4553
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1813233002

Review URL: https://codereview.chromium.org/1813233002
1 file changed