Allocation tracker: add separate entry for allocations via V8 API

When object is creating via native V8 API calls JS callstack is empty and the allocation is indistinguishable from say compiler allocations. This change adds a separate entry for such allocations.

Since FunctionInfo not necessarily corresponds to a heap object they are now referred to using their index in the list of all FunctionInfos.

BUG=chromium:277984
LOG=N
R=loislo@chromium.org, mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/177203002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19718 ce2b1a6d-e550-0410-aec6-3dcde31c8c00