Only mark the descriptor that is valid for the map in question. If this map
transitioned from a map with a different descriptor array (or has no back
pointer), mark all valid descriptors from the start.

This fixes the following memory leak: Map A shares a descriptor array
with map B. Map B adds constant function c that in its scope holds on to
an instance of B. If the descriptor array of A would keep all the shared
descriptors alive, including c, this keeps alive both A and c
indefinitely.

This CL also fixes a bug in descriptor array trimming. When trimming
descriptor arrays we need to trim off the slack as well (thus the entire
storage); and since we are trimming a descriptor array, we need to trim
* kDescriptorSize.

Review URL: https://chromiumcodereview.appspot.com/12084066

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13566 ce2b1a6d-e550-0410-aec6-3dcde31c8c00