This is the location where all the files under Source/wtf will be moved eventually. See the proposal and the design doc regarding the relocation project. For the project's progress, see bug 691465.
During the project, files in wtf/ are moved to platform/wtf incrementally, and redirection headers will be placed in wtf/. So nothing should break in the meantime. You can continue including WTF headers like #include "wtf/Xxx.h" till the next announce in blink-dev.