Have bindings layer assume and insist that all interface types are GCed.

third_party/WebKit/Source/core/testing/TypeConversions.idl

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 [
-    GarbageCollected,
 ] interface TypeConversions {
     attribute long testLong;
     [EnforceRange, ImplementedAs=testLong] attribute long testEnforceRangeLong;
     attribute unsigned long testUnsignedLong;
     [EnforceRange, ImplementedAs=testUnsignedLong] attribute unsigned long testEnforceRangeUnsignedLong;
 
     attribute long long testLongLong;
     [EnforceRange, ImplementedAs=testLongLong] attribute long long testEnforceRangeLongLong;
     attribute unsigned long long testUnsignedLongLong;
     [EnforceRange, ImplementedAs=testUnsignedLongLong] attribute unsigned long long testEnforceRangeUnsignedLongLong;
 
     attribute byte testByte;
     [EnforceRange, ImplementedAs=testByte] attribute byte testEnforceRangeByte;
     attribute octet testOctet;
     [EnforceRange, ImplementedAs=testOctet] attribute octet testEnforceRangeOctet;
 
     attribute short testShort;
     [EnforceRange, ImplementedAs=testShort] attribute short testEnforceRangeShort;
     attribute unsigned short testUnsignedShort;
     [EnforceRange, ImplementedAs=testUnsignedShort] attribute unsigned short testEnforceRangeUnsignedShort;
 
     attribute ByteString testByteString;
     void setTestByteString(ByteString byteString);
     [ImplementedAs=setTestByteString] void setTestByteStringDefaultNull(optional ByteString byteString = null);
 
     attribute USVString testUSVString;
     void setTestUSVString(USVString usvString);
     [ImplementedAs=setTestUSVString] void setTestUSVStringDefaultNull(optional USVString usvString = null);
 };