IDL compiler: Add a helper method for has_custom_getter and has_custom_setter
IDL compiler: Add a helper method for has_custom_getter and has_custom_setter

This refactoring is needed to make this CL (https://codereview.chromium.org/447523003/)
as smaller as possible. (I'm planning to land & revert the CL a couple of times,
so I want to make the CL as smaller as possible.)

Review URL: https://codereview.chromium.org/458443002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@179822 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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