Modify depot_tools instructions regarding %PATH%

Modify depot_tools instructions regarding %PATH%

The old depot_tools instructions insisted that depot_tools be the last
entry in the PATH environment variable, for reasons that are unclear and
may only have applied to Linux. Meanwhile, there is a gn issue that can
cause unnecessary building if depot_tools is not near the front of the
path - ahead of other Python installs. This change alters the
instructions for Windows.

This also removes some obsolete instructions regarding building Chrome
on Windows XP - Chrome doesn't even *run* on Windows XP anymore.

R=brucedawson@chromium.org
BUG=611087