These changes were necessary for me to build successfully with this in my
mozconfig:
ac_add_options --enable-debug
ac_add_options --disable-unified-build
ac_add_options --enable-profiling
While I'm here, I removed a duplicate include in InputTaskManager.h, and made
nsThreadPool.cpp include its own header first, to match Gecko common practice.
Differential Revision: https://phabricator.services.mozilla.com/D127503
