Add deployment target to build script.

Change-Id: Id020aaffb2a2175763b37e9b72414ae06a20dcf8
Reviewed-on: https://chromium-review.googlesource.com/c/1319407Reviewed-by: Hiroshi Ichikawa <ichikawa@chromium.org>
Commit-Queue: John Wu <jzw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605919}

ios/web_view/tools/build.py

@@ -233,6 +233,8 @@ def main():
   else:
     extra_gn_options += 'ios_web_view_enable_autofill=false '
   extra_gn_options += 'ios_web_view_output_name="%s" ' % output_name
+  # This is needed until all clients drop iOS 10 support.
+  extra_gn_options += 'ios_deployment_target="10.0" '
   # This prevents Breakpad from being included in the final binary to avoid
   # duplicate symbols with the client app.
   extra_gn_options += 'use_crash_key_stubs=true '