Use MigrateToMap instead of set_map to update the map of a JSObject.

Use MigrateToMap instead of set_map to update the map of a JSObject.
This is necessary to guarantee correct representation usage.
Some unhandlified code still needs to be handlified before we can push this
through fully.

BUG=
R=ishell@chromium.org

Committed: https://code.google.com/p/v8/source/detail?r=19844

[Toon Verwaest, 2014-03-11 10:00:37]

PTAL.
Prerequisite for introducing a mutable heap number instance type (and double
unboxing).

[Igor Sheludko, 2014-03-11 14:17:36]

lgtm with nits:

https://codereview.chromium.org/194783002/diff/1/src/objects.cc
File src/objects.cc (right):

https://codereview.chromium.org/194783002/diff/1/src/objects.cc#newcode2338
src/objects.cc:2338: int number_of_fields =
NumberOfFields(NumberOfOwnDescriptors());
NumberOfFields() would be also fine here.

https://codereview.chromium.org/194783002/diff/1/src/objects.cc#newcode2500
src/objects.cc:2500: int nof = limit == -1 ? NumberOfOwnDescriptors() : limit;
Do we really need this change?

[Toon Verwaest, 2014-03-11 14:45:10]

Addressed comment

[Toon Verwaest, 2014-03-12 13:18:33]

Committed patchset #2 manually as r19844 (presubmit successful).