Pin ios_deployment_target = "10.0" to unblock DEPS roll

Bug: libyuv:826
Change-Id: Ib47fa183615deaf870af9a9740ad9e8454f19941
Reviewed-on: https://chromium-review.googlesource.com/c/1312693Reviewed-by: Patrik Höglund <phoglund@chromium.org>
Commit-Queue: Oleh Prypin <oprypin@chromium.org>

.gn

@@ -36,4 +36,7 @@ default_args = {
 
   # LibYUV relies on Chromium's Android test infrastructure.
   use_cxx11_on_android = false
+
+  # https://bugs.chromium.org/p/libyuv/issues/detail?id=826
+  ios_deployment_target = "10.0"
 }