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We are trying to uprev the tcmalloc version used in Chromium.
As part of the transition to a new tcmalloc, we plan to run experiments
with the new tcmalloc on a small number of boards before we enable the
new version globally.

This CL adds a build flag to select between the two tcmalloc versions at
build time. By default, the old tcmalloc version is used.

BUG=724399,b:70905156

Change-Id: I6839bba721b0d2a5f3515e7346bff418b0155221
Reviewed-on: https://chromium-review.googlesource.com/1130672Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Albert J. Wong <ajwong@chromium.org>
Reviewed-by: Will Harris <wfh@chromium.org>
Commit-Queue: Gabriel Marin <gmx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577320}