gn/win: Don't strip trailing = on exec_script arguments.

BUG=588799

Review URL: https://codereview.chromium.org/1721883002

Cr-Original-Commit-Position: refs/heads/master@{#376983}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: baf2794a6e1adbd276deecd16637d4829a358ed8