Added ability to enable enhanced bookmarks related features from a finch experiment
Left "enable-override-bookmark-ui" out until Mike's CL finalizes

BUG=321393

Review URL: https://codereview.chromium.org/100803002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238858 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238858
Fix build/gyp_chromium for supplement.gypi files with no variables.

BUG=325989
TBR=brettw@chromium.org

Review URL: https://codereview.chromium.org/106073002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238859 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238859