Add libunwindstack to src/third_party

libunwindstack will be used to extend Stack Sampled Metrics (previously
known as the UMA sampling profiler) from Mac and Windows to Android.

A broader design doc for this effort is available at
http://bit.ly/2okOgOY.

The repo being included is available at http://bit.ly/33XnOtK. Because
libunwindstack is part of the broader Android platform/system/core repo,
it shares its license (Apache version 2.0). The platform/system/core
license can be found at http://bit.ly/34e5QmT.

eng-review@ has already given approval for this repo (see
http://bit.ly/2pNIshi, which is Google internal).

Bug: 991960
Change-Id: Ie950d7600d34147211fb0fce588179b6a63afc61
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1879990Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: Charlie Andrews <charliea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711274}

DEPS

@@ -1194,6 +1194,11 @@ deps = {
       'condition': 'checkout_linux',
   },
 
+  'src/third_party/libunwindstack': {
+      'url': Var('chromium_git') + '/chromium/src/third_party/libunwindstack.git' + '@' + '42c8c62f292a4f43f2dfa17626cc857a1eea29ef',
+      'condition': 'checkout_android',
+  },
+
   'src/third_party/libvpx/source/libvpx':
     Var('chromium_git') + '/webm/libvpx.git' + '@' +  '9b73e21c0df8e13259bc9af461a65229d57aeb49',
 

third_party/.gitignore

@@ -140,6 +140,7 @@
 /libsrtp
 /libsync/src
 /libupnp
+/libunwindstack
 /libvpx/source/libvpx
 /libwebm/source
 /libyuv