Don't allow official component builds

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index acc203c143b692ca031da0b66153847e653a366a..8996d99d0827c7035fdea81258ee466c62e38e0a 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -30,6 +30,12 @@ declare_args() {
   root_extra_deps = []
 }
 
+if (is_official_build) {
+  # An official (maximally optimized!) component (optimized for build times)
+  # build doesn't make sense and usually doesn't work.
+  assert(!is_component_build)
+}
+
 # This file defines the following five main targets:
 #
 # "both_gn_and_gyp" should list every root target (target that nothing else