The main impetus is to improve performance when --harmony-tostring
is enabled, thanks to using a generic property load instead of a
megamorphic IC.

This also reduces duplication, as the API function
v8::Object::ObjectProtoToString can share the runtime implementation.

The only functional change in this patch is to drop an accidental difference
between the JS and API implementations: the arguments object should toString
as "[object Arguments]". The JS side was corrected in
https://code.google.com/p/v8/source/detail?r=3279, but the API version was
missed in that patch.

BUG=chromium:555127, v8:3502
LOG=n
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_chromium_rel_ng;tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1509533003

Cr-Commit-Position: refs/heads/master@{#32777}