IDL: Add forEach() method to iterable<>/maplike<>/setlike<> interfaces

LayoutTests/webmidi/requestmidiaccess-expected.txt

 Tests navigator.requestMIDIAccess.
 
 On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
 
 
 PASS requestMIDIAccess() succeeded with access [object MIDIAccess].
 PASS access.sysexEnabled is defined.
 PASS access.sysexEnabled is false
 PASS access.inputs.size is 1
 PASS access.outputs.size is 1
 for (var input of inputs.values())
 PASS input.id is "MockInputID"
 PASS input.manufacturer is "MockInputManufacturer"
 PASS input.name is "MockInputName"
 PASS input.version is "MockInputVersion"
 for (var input of inputs.keys())
 PASS inputKey is "MockInputID"
 for (var input of inputs.entries())
 PASS entry[0] is inputKey
 PASS entry[1] is input
 for (var input of inputs)
 PASS entry[0] is inputKey
 PASS entry[1] is input
 PASS inputs.has('MockInputID') is true
 PASS inputs.has('MockOutputID') is false
 PASS inputs.get('MockInputID') is input
 PASS inputs.get('MockOutputID') is undefined.
+inputs.forEach(...)
+PASS forEachInput is input
+PASS forEachInputKey is inputKey
+PASS forEachMap is inputs
 for (var output of outputs.values())
 PASS output.id is "MockOutputID"
 PASS output.manufacturer is "MockOutputManufacturer"
 PASS output.name is "MockOutputName"
 PASS output.version is "MockOutputVersion"
 for (var output of outputs.keys())
 PASS outputKey is "MockOutputID"
 for (var output of outputs.entries())
 PASS entry[0] is outputKey
 PASS entry[1] is output
 for (var output of outputs)
 PASS entry[0] is outputKey
 PASS entry[1] is output
 PASS outputs.has('MockOutputID') is true
 PASS outputs.has('MockInputID') is false
 PASS outputs.get('MockOutputID') is output
 PASS outputs.get('MockInputID') is undefined.
+outputs.forEach(...)
+PASS forEachOutput is output
+PASS forEachOutputKey is outputKey
+PASS forEachMap is outputs
 PASS a note on message is sent without timestamp
 PASS a note on message is sent with timestamp
 PASS output.send([0xfff, 0x45, 0x7f]) threw exception TypeError: Failed to execute 'send' on 'MIDIOutput': The value at index 0 (4095) is greater than 0xFF..
 PASS output.send([0xf0, 0x45, 0xf7]) threw exception InvalidAccessError: Failed to execute 'send' on 'MIDIOutput': System exclusive message is not allowed at index 0 (240)..
 PASS requestMIDIAccess() was correctly blocked for System Exclusive access with error [object DOMError].
 PASS successfullyParsed is true
 
 TEST COMPLETE