HeapProfiler: replace pointer based matching algorithm with string matching algorithm for strings_ member.

pros: decreased snapshot size.
cons: increased serialization time.

I've tested the implementation on gmail 90mb heap.
I saw no speed degradation on the serialization step.
The snapshot size lost ~3% of its size. 100Mb -> 97Mb.

BUG=none
R=alph@chromium.org, yangguo@chromium.org

Review URL: https://codereview.chromium.org/24120006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16725 ce2b1a6d-e550-0410-aec6-3dcde31c8c00