Oilpan: move context menu providers to the heap.

Avoids keeping Persistent<> references on CustomContextMenuProvider.

R=haraken
BUG=340522

Review URL: https://codereview.chromium.org/665763002

git-svn-id: svn://svn.chromium.org/blink/trunk@183966 bbb929c8-8fbe-4397-9dbb-9b2b20218538