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A rough approximation of https://heycam.github.io/webidl/#es-union when
trying to convert an ES value to an IDL sequence or frozen array is:
1. Call the GetMethod abstract ES operation to retrieve the @@iterator
   property.
2. If it is not undefined, invoke
   https://heycam.github.io/webidl/#create-sequence-from-iterable with the
   ES value and the method obtained above.

So far, we were implementing it roughly like this:
1. Call the GetMethod abstract ES operation to retrieve the @@iterator
   property.
2. Invoke https://heycam.github.io/webidl/#es-sequence with the ES value,
   which disregards the method obtained in step 1 and goes through the whole
   process of calling GetMethod again and using that to create a sequence
   from an interable.

This is obviously not compliant with the spec, and the multiple calls to the
GetMethod abstract operation are user-visible, as can be seen in the test
case attached to the bug 1024388.

Properly fixing this requires changes to a few different classes and
functions:
* Make HasCallableIteratorSymbol(), which is called by the overload
  resolution algorithm implementation, be a small wrapper around
  GetEsIteratorMethod(). While HasCallableIteratorSymbol() precedes
  GetEsIteratorMethod(), the latter's implementation is more spec-compliant.
* Add a move constructor to ScriptIterator, as well as a static method that
  can create a ScriptIterator out of an ES value by invoking
  GetEsIteratorMethod() and GetEsIteratorWithMethod() underneath.
* Make NativeValueTraits<IDLSequence<T>>::ConvertSequenceSlow() take a
  ScriptIterator rather than a v8::Local<v8::Object>, which allows us to
  delegate getting an iterator out of an iterable object to the static
  ScriptIterator method mentioned above. This also helps reduce the size of
  the generated per-type NativeValueTraits<IDLSequence<T>> template code.
* Analogously, invoke ScriptIterator::FromIterable() in the union conversion
  code, and invoke a new NativeValueTraits<IDLSequence<T>>::NativeValue()
  overload that takes a ScriptIterator, so that we do not access the
  @@iterator property multiple times in the union conversion code.

Bug: 1024388
Change-Id: I47e6c0ca881e6e77f883ee78b5e1611138cdcc4a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1917169
Commit-Queue: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#716960}