clang/win: Let EGLDeviceTest.cpp build with -Wextra. In a Chromium build, this target is built as part of gpu, with the chromium_code warning settings applied. Make sure it builds in that config. BUG=567877 Change-Id: I7464538a259295d83d990853b06b9ba76b82889d Reviewed-on: https://chromium-review.googlesource.com/317360 Reviewed-by: Jamie Madill <jmadill@chromium.org> Tested-by: Nico Weber <thakis@chromium.org>