[Tracing] Make StackFrameDeduplicator take a range

Previously, |StackFrameDeduplicator::Insert| took a |Backtrace| which
contains a fixed size array. Taking [begin, end) pointers like the STL
APIs is more flexible. Taking templated [begin, end) iterators instead
of pointers would be even more flexible as it would allow passing in
backtraces from e.g. an |std::list|, but at this point there is no need
for that.

This is in preparation of a major change to |HeapDumpWriter|, which will
internally use pointer ranges instead of |Backtrace| structs. Making the
deduplicator take a range removes the need to copy a range back into a
|Backtrace| struct for deduplication.

As a drive-by fix, reduce boilerplate in |StackFrameDeduplicator| tests
and add a test to ensure that trailing null pointers are stripped.

BUG=524631

Review URL: https://codereview.chromium.org/1500693002

Cr-Commit-Position: refs/heads/master@{#363528}

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