Add support for ES6 Symbol in heap profiler

Heap profiler will create a node with name Symbol and type kSymbol.

BUG=chromium:376194
LOG=Y
R=loislo@chromium.org, yangguo@chromium.org

Review URL: https://codereview.chromium.org/290013004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21433 ce2b1a6d-e550-0410-aec6-3dcde31c8c00