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The actual physical memory usage of tcmalloc cannot be obtained by
GetNumericProperty. This accounts for the current_allocated_bytes,
fragmentation and malloc metadata, and excludes the unmapped memory
regions. This helps the user to understand how much memory is actually
being used for the allocations that were made.

This change is done only in chromium tree since the
metadata_unmapped_bytes is added only in the chromium's tcmalloc in CL:
www.crrev.com/10499004.

This is to be used by malloc dump provider for tracing.

BUG=546491

Review URL: https://codereview.chromium.org/1410353005

Cr-Commit-Position: refs/heads/master@{#357155}