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There is callback map in extension function dispatcher, which store
pointers on callback wrappers. But it force to remember to delete
wrapper after erase from map. This may lead to memory leak.
To prevent it, raw pointer in map was replaced by unique_ptr (like in
other callback map in same class).

BUG=None
R=finnur

Review-Url: https://codereview.chromium.org/2850533002
Cr-Commit-Position: refs/heads/master@{#467932}