build: Add Python 3 support (see issue #2856)

tools/gypi_to_gn.py

@@ -93,6 +93,8 @@ the input will be replaced with "bar":
 
 """
 
+from __future__ import absolute_import
+from __future__ import print_function
 from optparse import OptionParser
 import os
 import sys
@@ -100,7 +102,7 @@ import sys
 try:
   # May already be in the import path.
   import gn_helpers
-except ImportError, e:
+except ImportError as e:
   # Add src/build to import path.
   cef_dir = os.path.abspath(os.path.join(os.path.dirname(__file__), os.pardir))
   src_dir = os.path.abspath(os.path.join(cef_dir, os.pardir))
@@ -112,10 +114,10 @@ def LoadPythonDictionary(path):
   file_string = open(path).read()
   try:
     file_data = eval(file_string, {'__builtins__': None}, None)
-  except SyntaxError, e:
+  except SyntaxError as e:
     e.filename = path
     raise
-  except Exception, e:
+  except Exception as e:
     raise Exception("Unexpected error while reading %s: %s" % (path, str(e)))
 
   assert isinstance(file_data, dict), "%s does not eval to a dictionary" % path
@@ -194,11 +196,11 @@ def main():
       data[key[:-1]] = data[key]
       del data[key]
 
-  print gn_helpers.ToGNString(data)
+  print(gn_helpers.ToGNString(data))
 
 if __name__ == '__main__':
   try:
     main()
-  except Exception, e:
-    print str(e)
+  except Exception as e:
+    print(str(e))
     sys.exit(1)