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This is necessary since there's now 3 different ABIs for libc++: ndk1, 1, and
Cr.  The ndk1 and 1 ABIs already had prebuilts, but the Cr ABI was introduced to
fix an issue with Android component builds.  All three are necessary right now,
but the ndk1 prebuilt can be removed once use_custom_libcxx=true is set on all
Android builds.

Internal CL that updates docs:
https://chrome-internal-review.googlesource.com/c/clank/internal/apps/+/767848

BUG=767901
R=bajones
CC=​thakis

Change-Id: Ib8d361152a5efb169a42c370fa4e6a80025b7af5
Reviewed-on: https://chromium-review.googlesource.com/c/1395186Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#620027}