[Tracing] Preallocate vector in HeapDumpWriter
[Tracing] Preallocate vector in HeapDumpWriter

The size of the vector is known in advance, so there is no need for
reallocations or wasted space.

BUG=524631

Review URL: https://codereview.chromium.org/1442983002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#359819}

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