Add a second-order Dictionary::convert() function template.
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: bf5cd6ccde378db821296732a091cfbcf5285fbd@167271 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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