Remove HeapSnapshotsCollection class

The class added unecessary level of indirection to the heap profiler implementation. I merged some of its implementation into HeapProfiler and deleted the rest. This refactoring is also a prerequisite for keeping HeapObjectsMap alive even when all snapshots are deleted.

BUG=None
LOG=N
R=alph@chromium.org, mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/101393002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18221 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Enable new string add.

BUG=v8:2990
LOG=n
R=hpayer@chromium.org

Review URL: https://codereview.chromium.org/100223003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18222 ce2b1a6d-e550-0410-aec6-3dcde31c8c00