Move CallbackBase::polymorphic_invoke_ into BindStateBase

Each Callback instance had its own polymorphic_invoke, that doubles the
size of Callback, and its initialization gains 40kB of the stripped
binary size of Chrome on Linux.

This CL moves polymorphic_invoke_ into BindStateBase to share it among
Callback.

Review-Url: https://codereview.chromium.org/2317563002
Cr-Commit-Position: refs/heads/master@{#417197}
6 files changed