Don't rely on implicit lookup of member names in dependent base classes

Don't rely on implicit lookup of member names in dependent base classes

Summary:
The standard says this shouldn't be done, but GCC, Clang, and MSVC all do, at least, MSVC does if you don't pass the `/permissive-` switch to disable this non-compliant lookup.
This just qualifies it with `this->` which solves the problem.

MSVC plans to eventually (some time after the 2017 release) enable `/permissive-` by default, so let's start compiling with it now.

Reviewed By: yfeldblum

Differential Revision: D4200725

fbshipit-source-id: 8545c6bcda9ba6006733c15f668805cb1260517a