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.

Review URL: https://chromiumcodereview.appspot.com/10445009

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@11651 ce2b1a6d-e550-0410-aec6-3dcde31c8c00