Refactor gyp_chromium to ease reuse.

Moves the new Visual Studio 2013 toolchain logic and
download into a helper function. That way it can easily be used
by client projects that have their own gyp_* files.
See http://review.webrtc.org/7759004/ for an example.

NOTRY=True
TEST=Successful local execution on Mac and Windows.

Review URL: https://codereview.chromium.org/185623004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254626 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254626