Fix DEPOT_TOOLS_WIN_TOOLCHAIN=0 being ignored
Fix DEPOT_TOOLS_WIN_TOOLCHAIN=0 being ignored

Only use the existence of the win_toolchain.json file as a signal when
we're using the depot_tools toolchain (based on the value of
DEPOT_TOOLS_WIN_TOOLCHAIN).

R=brucedawson@chromium.org
BUG=523154

Review URL: https://codereview.chromium.org/1315003002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#345472}

ltilve/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend