Get rid of ui_cocoa_third_party_toolkits target from ui.gyp

Instead we add a gyp file in third_party/google_toolbox_for_mac to build
the sources from third_party/GTM. This is the third and last patch in
this series.

BUG=299841
TEST=None
R=mark@chromium.org,ben@chromium.org
TBR=ben

Review URL: https://codereview.chromium.org/107933006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241471 0039d316-1c4b-4281-b951-d872f2087c98
Update .DEPS.git

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241472 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241472