When non-Googlers try to build Chromium for the first time they get an
error saying:

  Please follow the instructions at https://chromium.googlesource.com/chromium/src/+/master/docs/windows_build_instructions.md

However those instructions are quite long and it's not obvious to some
users which part of the instructions they have forgotten. The most
likely is that they didn't set DEPOT_TOOLS_WIN_TOOLCHAIN=0, so this
changes the error message to suggest that directly.

Auto-detecting the correct state for DEPOT_TOOLS_WIN_TOOLCHAIN was
tried in crrev.com/c/1374833 but deemed impractical.

Bug: 907300, angleproject:2712
Change-Id: I45a9f59babe90bbe2137d49c555884d7bbbcf170
Reviewed-on: https://chromium-review.googlesource.com/c/1382942Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>