Don't allow official component builds

Official component builds generally don't work, and they are logically
contradictory, so it is better to prevent them with an assert in order
to save developers from this mistake.

Review-Url: https://codereview.chromium.org/2301213002
Cr-Commit-Position: refs/heads/master@{#416321}

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