Before these changes, ContentChild.h was the #9 most expensive header to include:
217505 ms: /home/simon/work/fuzzy/obj-x86_64-pc-linux-gnu-optimized/dist/include/mozilla/dom/ContentChild.h (included 105 times, avg 2071 ms)
After these changes, it is no longer among the top 30 most expensive headers.
Some missing includes in other files are added along.