Support read-only attributes in code generator in python

This is one of migrating binding code generator
from perl to python (with jinja template engine).

With this patch, generator in python newly supports
22 IDLs which contains only readonly attribute.

R=haraken@chromium.org
BUG=262365
TEST=no tests. refactoring.

Review URL: https://chromiumcodereview.appspot.com/21784004

git-svn-id: svn://svn.chromium.org/blink/trunk@155517 bbb929c8-8fbe-4397-9dbb-9b2b20218538