port 72bc4b5c (r30557)

original commit message:

    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.

BUG=

Review URL: https://codereview.chromium.org/1331523002

Cr-Commit-Position: refs/heads/master@{#30610}