Update Windows build instructions.

This updates the windows build instructions to clarify how
you need to adjust your PATH environment variable to pick
up depot_tools, and to note that NTFS is required.

R=brucedawson@chromium.org, danakj@chromium.org
BUG=611087

Review-Url: https://codereview.chromium.org/2630993002
Cr-Commit-Position: refs/heads/master@{#444773}
1 file changed