Allow building Chrome with local VC++ 2017 install

This updates the toolchain scripts enough so that it is possible to
build Chrome with VC++ 2017 if DEPOT_TOOLS_WIN_TOOLCHAIN=0 and
GYP_MSVS_VERSION=2017, followed by gclient runhooks.

There are still some local patches needed to make it work, and
treat_warnings_as_errors = false and is_component_build = false in
args.gn may be needed initially, but my local builds are working.

The toolchain code should be updated eventually to use COM to locate
the VC++ install location, instead of the hard-coded path which is being
used as a temporary measure in this change.

R=scottmg@chromium.org
BUG=683729

Review-Url: https://codereview.chromium.org/2653473002
Cr-Original-Commit-Position: refs/heads/master@{#445329}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: adddab40528d1fb6dd94c0e4d2bc794ffccf194a
1 file changed