Add GN build for parts of gpu and ozone.

tools/gn/builder.cc

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "tools/gn/builder.h"
 
 #include "tools/gn/config.h"
 #include "tools/gn/err.h"
 #include "tools/gn/loader.h"
 #include "tools/gn/scheduler.h"
@@ skipping 429 matching lines @@
     for (size_t dep_i = 0; dep_i < deps.size(); dep_i++) {
       if (configs[config_i].label == deps[dep_i].label) {
         DCHECK(deps[dep_i].ptr);  // Should already be resolved.
         configs[config_i].ptr = deps[dep_i].ptr;
         break;
       }
     }
     if (!configs[config_i].ptr) {
       *err = Err(target->defined_from(),
           "Target in forward_dependent_configs_from was not listed in the deps",
-          "The target \"" + configs[config_i].label.GetUserVisibleName(false) +
-          "\"\n was not present in the deps. This thing is used to forward\n"
-          "configs from direct dependents.");
+          "This target has a forward_dependent_configs_from entry that was "
+          "not present in\nthe deps. A target can only forward things it "
+          "depends on. It was forwarding:\n  " +
+          configs[config_i].label.GetUserVisibleName(false));
       return false;
     }
   }
   return true;
 }
 
 std::string Builder::CheckForCircularDependencies(
     const std::vector<const BuilderRecord*>& bad_records) const {
   std::vector<const BuilderRecord*> cycle;
   if (!RecursiveFindCycle(bad_records[0], &cycle))
     return std::string();  // Didn't find a cycle, something else is wrong.
 
   // Walk backwards since the dependency arrows point in the reverse direction.
   std::string ret;
   for (int i = static_cast<int>(cycle.size()) - 1; i >= 0; i--) {
     ret += "  " + cycle[i]->label().GetUserVisibleName(false);
     if (i != 0)
       ret += " ->\n";
   }
 
   return ret;
 }