Make the Windows multi_dll build work in GN

This patch still keeps the flag disabled, I will toggle that in a separate patch to reduce risk.

CQ_EXTRA_TRYBOTS=tryserver.chromium.linux:android_chromium_gn_compile_dbg,android_chromium_gn_compile_rel;tryserver.chromium.win:win8_chromium_gn_rel,win8_chromium_gn_dbg;tryserver.chromium.mac:mac_chromium_gn_rel,mac_chromium_gn_dbg

Review URL: https://codereview.chromium.org/1122613002

Cr-Commit-Position: refs/heads/master@{#328031}
8 files changed