Add a second-order Dictionary::convert() function template.

Avoid source code duplication of convert() method bodies for the
parameterized pointer types.

R=
BUG=

Review URL: https://codereview.chromium.org/168483002

git-svn-id: svn://svn.chromium.org/blink/trunk@167271 bbb929c8-8fbe-4397-9dbb-9b2b20218538