win: Find and use toolchain installed by depot_tools

The toolchain directory inside depot_tools will only exist if
"set DEPOT_TOOLS_WIN_TOOLCHAIN=1" so this is currently a no-op.

It will break the "dogfood" of 'GYP_MSVS_USE_SYSTEM_TOOLCHAIN'
now renamed per above, but I think only a few people tested
that (cc'd here).

R=dpranke@chromium.org
BUG=323300, 326357

Review URL: https://codereview.chromium.org/137573002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245412 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245412