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It retrieves the currently collected native heap sampling snapshot.
The renderer process had to be started with --sampling-heap-profiler flag to
obtain the snapshot.

BUG=803276

Change-Id: I26ffe5e366abb33245d2f83ffa02bd4350737b7a
Reviewed-on: https://chromium-review.googlesource.com/882290
Commit-Queue: Alexei Filippov <alph@chromium.org>
Reviewed-by: Pavel Feldman <pfeldman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532233}