GN: build Lua tools when skia_use_lua.

GN: build Lua tools when skia_use_lua.

When we opt into Lua, this builds SampleLua into SampleApp, and the lua_app and lua_pictures tools.

I've tested this builds with and without skia_use_system_lua on my Mac laptop and Linux desktop.

I've made lua_pictures.cpp's flags static to avoid conflicts with flags in SkCommonFlags.cpp.

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4699

DOCS_PREVIEW= https://skia.org/?cl=4699

Change-Id: I8176fd51d8a38746e7d730cfcce66da42b9a015a
Reviewed-on: https://skia-review.googlesource.com/4699
Reviewed-by: Hal Canary <halcanary@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>