Mojo: Add a Mojo-specific multiprocess test base class.

This sets up (most of) the infrastructure that'll be needed to set up a
PlatformChannel between parent and child processes.

R=darin@chromium.org, darin

Review URL: https://codereview.chromium.org/99473007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238425 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238425
gyp_chromium integration for automatic toolchain

When opting in via GYP_MSVS_USE_SYSTEM_TOOLCHAIN=0, set
environment variables that tell gyp how to find the automatic
toolchain in third_party/win_toolchain.

R=maruel@chromium.org
BUG=323300

Review URL: https://codereview.chromium.org/100613002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238426 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238426