Replace HeapNumber as doublebox with an explicit MutableHeapNumber.

Replace HeapNumber as doublebox with an explicit MutableHeapNumber.

R=verwaest@chromium.org

Committed: https://code.google.com/p/v8/source/detail?r=22082

[Igor Sheludko, 2014-06-25 09:48:03]

PTAL. This is a rebased version of your CL
https://codereview.chromium.org/211333002/ plus minor changes and ports for
other platforms.

[Toon Verwaest, 2014-06-25 12:04:25]

Could you add tests that reading doubles after transitioning works? Basically
just look at all different cases where we do MigrateToMap, and ensure that they
actually work. E.g., elements transitions after having a double field should not
leak a mutable heap number.

[Igor Sheludko, 2014-06-27 09:30:22]

Added a brute-force test that covers most of the cases when Map::MigrateToMap()
is called.

[Toon Verwaest, 2014-06-30 09:27:33]

lgtm

[Igor Sheludko, 2014-06-30 09:45:23]

Committed patchset #3 manually as r22082 (presubmit successful).