Allow GCC 4.9 to compile Chromium

In order to implicit cast an lvalue to an rvalue when returning
from a function, the return type and type of variable in the return
statement previously had to be exactly the same. When this was not
the case, std::move was required. For instance, when returning a
std::unique_ptr<Derived> variable in a function with a
std::unique_ptr<Base> return type, std::move is required.

DR 1579 changed this, and allows for implicitly converting
to the return type, if the return type has a constructor(T&&), where
T is the type of the local variable being returned. DR 1579 was
implemented in GCC 5, but not in GCC 4.9 and below. By explicitly
qualifying the local variable with std::move, we allow for compiling
with GCC 4.9 and incur no performance penalty. The code is still
absolutely correct to the word of C++11.

BUG=chromium:682965

See also:
* https://bugs.gentoo.org/show_bug.cgi?id=600288
* https://stackoverflow.com/questions/22018115/converting-stdunique-ptrderived-to-stdunique-ptrbase#comment33375875_22018521
* http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n3833.html#1579

Review-Url: https://codereview.webrtc.org/2642053003
Cr-Commit-Position: refs/heads/master@{#16175}

AUTHORS

@@ -24,6 +24,7 @@ Manish Jethani <manish.jethani@gmail.com>
 Martin Storsjo <martin@martin.st>
 Matthias Liebig <matthias.gcode@gmail.com>
 Maxim Potapov <vopatop.skam@gmail.com>
+Mike Gilbert <floppymaster@gmail.com>
 Pali Rohar
 Paul Kapustin <pkapustin@gmail.com>
 Philipp Hancke <philipp.hancke@googlemail.com>