tl;dr: Auto-generated IDL union classes are now GarbageCollected.
Changes that affect Blink developers:
1. The new IDL union classes are now GarbageCollected.
old_union.Foo() is replaced with new_union->Foo().
2. Generated file paths are different.
3. Generated class names are different.
4. Minor changes
4-a. (A or B) and (B or A) in *.idl generated separate types (AOrB and BOrA). Now both IDL definitions generate a single V8UnionAOrB.
4-b. (A? or B) and (A or B) in *.idl generated a single type AOrB. Now they generate separate types V8UnionAOrBOrNull and V8UnionAOrB.
4-c. New switch-statement-friendly API, GetContentType()
IDL dictionaries are not yet supported by the new Web IDL compiler. IDL dictionaries are generated by the old code generator and do not use the new IDL unions yet, so IDL dictionary members of type IDL unions use the old IDL union types for the time being. IDL dictionaries are planned to be upgraded soon.