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This is a major change to |HeapDumpWriter| which enables it to dump the
format described in https://goo.gl/KY7zVE. This will allow the heap
profiler to break down the heap by type or backtrace, but also
combinations of those.

This changes the interface of |HeapDumpWriter| slightly from a two-step
process (insert allocations, dump json) to a three-step process (insert
allocations, compute what to dump, convert to json). The main reason for
doing this is testability. Aggregation and json conversion are now
completely orthogonal. This means that the aggregation algorithm can be
tested without having to parse the json first, and the json conversion
is simpler to test too.

This is part of the heap profiler in chrome://tracing.

BUG=524631

Review URL: https://codereview.chromium.org/1494293005

Cr-Commit-Position: refs/heads/master@{#364444}