Now that Chromium is switching to use_gtk3=true by default, it is
unnecessary to build libgtk3ui from gn_all.

In addition, this CL also guards libgtk2ui with !use_gtk3 so that
gtk+-2.0.pc is not required during gn gen (and so gtk+-3.0.pc is not
required for a gtk2 build).

BUG=668446
R=dpranke@chromium.org,erg@chromium.org

Review-Url: https://codereview.chromium.org/2695823006
Cr-Commit-Position: refs/heads/master@{#450611}