Extend accepted functors on base::Bind{Once,Repeating}

This CL allows empty functors on base::BindOnce and base::Repeating, and
adds base::BindLambdaForTesting that is a test-only variant of Bind that
binds any callable object to a RepeatingCallback.

Bug: 789805
Change-Id: I7fa34acd2a6b79e8c26d3f6ded67e91bd8d6b966
Reviewed-on: https://chromium-review.googlesource.com/798817
Commit-Queue: Taiju Tsuiki <[email protected]>
Reviewed-by: Daniel Cheng <[email protected]>
Cr-Commit-Position: refs/heads/master@{#521192}
5 files changed