The flag came in with a merge conflict from JDK-8333542.

I don't think it added an alignment gap but this could use further investigation.

	u2                         _nest_host_index;     /*   304     2 */
	u2                         _this_class_index;    /*   306     2 */
	u2                         _static_oop_field_count; /*   308     2 */
	volatile u2                _idnum_allocated_count; /*   310     2 */
	bool                       _is_marked_dependent; /*   312     1 */
	volatile enum ClassState   _init_state;          /*   313     1 */
	u1                         _reference_type;      /*   314     1 */

	/* XXX 1 byte hole, try to pack */

	class InstanceKlassFlags  _misc_flags;           /*   316     4 */

	/* XXX last struct has 1 byte of padding */
	/* --- cacheline 5 boundary (320 bytes) --- */
	volatile class JavaThread *  _init_thread;       /*   320     8 */


With field removed.

	u2                         _nest_host_index;     /*   304     2 */
	u2                         _this_class_index;    /*   306     2 */
	u2                         _static_oop_field_count; /*   308     2 */
	volatile u2                _idnum_allocated_count; /*   310     2 */
	volatile enum ClassState   _init_state;          /*   312     1 */
	u1                         _reference_type;      /*   313     1 */
	class InstanceKlassFlags  _misc_flags;           /*   314     4 */

	/* XXX last struct has 1 byte of padding */
	/* XXX 2 bytes hole, try to pack */

	/* --- cacheline 5 boundary (320 bytes) --- */
	volatile class JavaThread *  _init_thread;       /*   320     8 */


From pahole.  There's nothing else to pack in those two bytes.