Enable compiling GN by default.

tools/gn/err.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/err.h"
 
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/string_util.h"
 #include "tools/gn/filesystem_utils.h"
 #include "tools/gn/input_file.h"
@@ skipping 23 matching lines @@
   // Watch out, the char offsets in the location are 1-based, so we have to
   // subtract 1.
   int begin_char;
   if (range.begin().line_number() < line_number)
     begin_char = 0;
   else
     begin_char = range.begin().char_offset() - 1;
 
   int end_char;
   if (range.end().line_number() > line_number)
-    end_char = line->size();  // Ending is non-inclusive.
+    end_char = static_cast<int>(line->size());  // Ending is non-inclusive.
   else
     end_char = range.end().char_offset() - 1;
 
   CHECK(end_char >= begin_char);
   CHECK(begin_char >= 0 && begin_char <= static_cast<int>(line->size()));
   CHECK(end_char >= 0 && end_char <= static_cast<int>(line->size()));
   for (int i = begin_char; i < end_char; i++)
     line->at(i) = '-';
 }
 
@@ skipping 127 matching lines @@
   }
 
   // Optional help text.
   if (!help_text_.empty())
     OutputString(help_text_ + "\n");
 
   // Sub errors.
   for (size_t i = 0; i < sub_errs_.size(); i++)
     sub_errs_[i].InternalPrintToStdout(true);
 }