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This is needed for the "Build From Tarball" bot which unbundles
libraries.  Some of the libraries we bundle are very close to ToT,
and the build would fail if we were using an out-of-date version
without the latest features.

Sid allows us to have everything we need, and it doesn't matter
that the libraries are unstable since we don't actually run
anything against them.

R=thestig,dpranke

Change-Id: I0c42113b2e1bdc910230742cd22ad0681699291a
Reviewed-on: https://chromium-review.googlesource.com/823563Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#536904}