Instead of having a nested enum within a struct, use the macro
AK_ENUM_BITWISE_OPERATORS to add all the convienent has_flag free
functions and such for ease of use.
This remained undetected for a long time as HeaderCheck is disabled by
default. This commit makes the following file compile again:
// file: compile_me.cpp
#include <LibWeb/HTML/CrossOrigin/CrossOriginOpenerPolicy.h>
// That's it, this was enough to cause a compilation error.
Likewise for most other files touched by this commit.
