Replace generic Dictionary with DictionaryPluginPlaceholderOptions

Dictionary.h has some problems (it cannot handle ExceptionState,
difficult to understand). We want to reduce Dictionary uses
in core/modules so that make it more maintainable.

Tests under fast/plugins cover this change.

BUG=403150

Review URL: https://codereview.chromium.org/720673002

git-svn-id: svn://svn.chromium.org/blink/trunk@185194 bbb929c8-8fbe-4397-9dbb-9b2b20218538