Split Dictionary's get and convert into DictionaryHelper.

Dictionary's get and convert depends on both core and modules code.
So the get and convert logic is split into DictionaryHelper.

DictionaryHelper is a class which has static template methods: get and convert:
- the methods are implemented in DictionaryHelperForCore.cpp and DictionaryHelperForModules.cpp.
- DictionaryHelperForCore.cpp has get and convert for core code.
- DictionaryHelperForModules.cpp has get and convert for modules code.

BUG=358074

Review URL: https://codereview.chromium.org/373423002

git-svn-id: svn://svn.chromium.org/blink/trunk@177908 bbb929c8-8fbe-4397-9dbb-9b2b20218538