Calling a generator function returns a generator object

Calling a generator function returns a generator object

* src/heap.h:
* src/heap.cc:
* src/objects-debug.cc:
* src/objects-inl.h:
* src/objects-printer.cc:
* src/objects-visiting.cc:
* src/objects.cc:
* src/objects.h: Define a new object type, JSGeneratorObject.

* src/factory.h:
* src/factory.cc (NewFunctionFromSharedFunctionInfo): Generator function
  inital maps construct the new JS_GENERATOR_OBJECT_TYPE objects, not
  generic JSObjects.

* src/runtime.h:
* src/runtime.cc (Runtime_CreateJSGeneratorObject):
* src/arm/full-codegen-arm.cc (Generate):
* src/ia32/full-codegen-ia32.cc (Generate):
* src/x64/full-codegen-x64.cc (Generate): Before visiting generator
  bodies, arrange to construct and return a generator object.

* test/mjsunit/harmony/generators-objects.js: Add tests for the
  properties and prototype of generator objects.

BUG=v8:2355
TEST=mjsunit/harmony/generators-objects

Review URL: https://codereview.chromium.org/13542002

Patch from Andy Wingo <wingo@igalia.com>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@14264 ce2b1a6d-e550-0410-aec6-3dcde31c8c00