[es6] Introduce a dedicated JSIteratorResult type.

[es6] Introduce a dedicated JSIteratorResult type.

Use a single JSIteratorResult type for all implementation provided
iterator results (i.e. the String, Array and collection iterators,
and also for generators).  This removes one source of unnecessary
polymorphism in for-of loops.  It is accomplished by a new intrinsic
%_CreateIterResultObject() that should be used to create iterator
result objects from JavaScript builtins (there's a matching factory
method for C++ code).

Also restructure the %StringIteratorPrototype%.next() and
%ArrayIteratorPrototype%.next() functions to be a bit more friendly
to optimizing compilers.

R=ishell@chromium.org

Review URL: https://codereview.chromium.org/1302173007

Cr-Commit-Position: refs/heads/master@{#30557}