Clang and Chrome build with different versions of clang which support
different warning flags.  Ignore unknown warning flags when building in
Android.

Bug: b:142320172
Change-Id: I35eb424aa5a146fec7dabf9f3aa71bd6dd9d8f50
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2112282Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Geoff Lang <geofflang@chromium.org>