Explicitly don't support the make gyp generator.

Provide a friendly error message in gyp_chromium.

R=thakis@chromium.org
BUG=348686

Review URL: https://codereview.chromium.org/184823011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254721 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254721