IDL: Avoid unused variables when no member is defined in an IDL dictionary
IDL: Avoid unused variables when no member is defined in an IDL dictionary

When an empty IDL dictionary is defined, no conversion occurs in its
toImpl(), causing unused variables errors. Declare local variables
only when dictionaries have members.

BUG=321462

Review URL: https://codereview.chromium.org/770523003

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@186409 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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