Bug 1730774 part 1: Give EnumeratedRange.h an include for Assertions.h, since it includes MOZ_ASSERT statements. r=TYLin

Without this patch, these MOZ_ASSERT statements cause build errors, if I build
the layout/generic directory in non-unified mode.

(Instead of this patch, we could also hypothetically sprinkle individual
#includes for Assertions.h around to all the downstream files; but it's simpler
and more direct to just put it here in the file that contains the MOZ_ASSERT
statements in question. Note that many other MFBT headers also have includes
for Assertions.h, so it seems reasonable to include it here as well.)

Differential Revision: https://phabricator.services.mozilla.com/D125610

mfbt/EnumeratedRange.h

@@ -23,6 +23,7 @@
 #include <limits>
 #include <type_traits>
 
+#include "mozilla/Assertions.h"
 #include "mozilla/ReverseIterator.h"
 
 namespace mozilla {