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This patch makes WTF::Function an alias of base::OnceCallback. After
this, all the single-threaded callbacks in Blink will become base
callbacks.

This patch doesn't remove the WTF::Funtion type nor ConvertToBase-
Callback() function. The use sites of those names will be removed
individually.

Bug: 771087
Change-Id: I15e7758c29bb9f63b7db7ad7aba6c9cb2d9ae7d1
Reviewed-on: https://chromium-review.googlesource.com/807672Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Taiju Tsuiki <tzik@chromium.org>
Commit-Queue: Yuta Kitamura <yutak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521649}