Add build flags to disable desktop gl backend This change saves about 100KB on Android Bug: angleproject:5490 Change-Id: Ieca0e597423f74a917197f381560af5409a9e874 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2601341 Reviewed-by: Jonah Ryan-Davis <jonahr@google.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Peng Huang <penghuang@chromium.org>