Automatically generated with:
cp dom/.clang-format xpcom/ && mach format xpcom/**.{cpp,h}
Manual changes:
* nsTDependentSubstring.cpp was relying on include order, prevent that
from being changed by clang-format.
* Missing nsAtom.h include from nsGkAtoms.h
* nsWindowsHelpers.h needed to keep the include order
* MemoryPressureLevelMac.h needed mozilla/Attributes.h
Differential Revision: https://phabricator.services.mozilla.com/D311687