Fix naming in binding code generator (contents => context, no generate_)

This fixes a long-standing typo/misnaming in the CG:
The dict of variables Jinja uses to "render" a template is called the
"context", per API reference:
http://jinja.pocoo.org/docs/api/#jinja2.Template.render

However, we initially called this "contents".

Also, "generate" should only be used for "producing text",
which is how we use it in other build scripts,
not "compute values";
this is analogous to avoiding "get_" prefixes.

This fixes the names, making it rather more consistent and readable.
Compare for instance:
'methods': [generate_method(operation)
            for operation in callback_interface.operations],
with corrected:
'methods': [method_context(operation)
            for operation in callback_interface.operations],

Also fixes one old type in callback_interface
(missing "is_" on boolean).

R=haraken

Review URL: https://codereview.chromium.org/352483002

git-svn-id: svn://svn.chromium.org/blink/trunk@176737 bbb929c8-8fbe-4397-9dbb-9b2b20218538