Oilpan: Move core/plugins/ to oilpan's heap

This CL moves DOMMimeType, DOMMimeTypeArray, DOMPlugin, DOMPluginArray to oilpan's heap.
This CL does not move platform/plugins/PluginData to oilpan's heap since PluginData is exposed to outside.

BUG=340522

Review URL: https://codereview.chromium.org/170103002

git-svn-id: svn://svn.chromium.org/blink/trunk@167304 bbb929c8-8fbe-4397-9dbb-9b2b20218538