media/gpu/vaapi: Add the assertion that media/gpu/vaapi is built only for Linux and ChromeOS

VA-API is a linux-specific API. This CL adds the assertion that lets gn fail
if media/gpu/vaapi is built for neither Linux nor ChromeOS platform.

Bug: 1000575
Test: compile chrome
Change-Id: I71836047fe69bff28640297e58415983a97d6760
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1847591Reviewed-by: Dan Sanders <sandersd@chromium.org>
Commit-Queue: Hirokazu Honda <hiroh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#721268}

media/gpu/vaapi/BUILD.gn

@@ -10,6 +10,7 @@ import("//tools/generate_stubs/rules.gni")
 import("//ui/gl/features.gni")
 import("//ui/ozone/ozone.gni")
 
+assert(is_linux)
 assert(use_vaapi)
 
 generate_stubs("libva_stubs") {