Re-land: Use map transitions when defining accessor properties.

Re-land: Use map transitions when defining accessor properties.

This is basically r11496, with the following changes:

 * Set back pointers in maps (cherry-picked from r11528)

 * Fixed size calculation in CopyInsert, as proposed by mstarzinger/rossberg

 * DefineFastAccessor uses GetCallbackObject instead of GetValue (for __proto__)

 * Put the code under a new flag, which is disabled by default

 * Cut down the corresponding regression test

 * Adapted bootup memory test, we actually only need a bit more memory on 64bit without snapshots, which can easily explained by more live maps lying around. Note that the snapshot variants are back to their previous limits.

Next steps: Investigate any performance degradationswith the flag enabled, and finally remove the flag when things are OK. Furthermore, GetCallbackObject should be merged into GetValue, the distinction is confusing and error-prone.

Committed: https://code.google.com/p/v8/source/detail?r=11651

[Michael Starzinger, 2012-05-24 10:03:59]

LGTM.