Allow implicitly converting a lambda to mojo::Callback using C++11

Allow implicitly converting a lambda to mojo::Callback using C++11

This uses C++11 type traits to check if a type is convertible to an
std::function and if so uses an appropriate adaptor for mojo::Callback.
This replaces the fairly hacky operator() detection that was in place.

R=viettrungluu@chromium.org

Committed: https://chromium.googlesource.com/external/mojo/+/bdf32e44b287cb2ddb5509d37528296b5d08a1a6

[jamesr, 2015-10-09 18:20:46]

jamesr@chromium.org changed reviewers:
+ viettrungluu@chromium.org

[viettrungluu, 2015-10-09 21:10:21]

bots seem awfully red

[jamesr, 2015-10-13 22:22:05]

Compile failure was due to other users of mojo::internal::Conditional, which are
gone now. PTAL

[viettrungluu, 2015-10-13 23:06:00]

lgtm

[jamesr, 2015-10-14 23:37:48]

Committed patchset #2 (id:20001) manually as
bdf32e44b287cb2ddb5509d37528296b5d08a1a6 (presubmit successful).