GN: components as static library instead of source set

This also puts the .a in the top-level of the ouptut directory, just
like we do for .so files.  It's less required than for .sos, but it
does make it easier to find the .a.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2197633002

Review-Url: https://codereview.chromium.org/2197633002
diff --git a/gn/BUILDCONFIG.gn b/gn/BUILDCONFIG.gn
index 31056bd..681b15b 100644
--- a/gn/BUILDCONFIG.gn
+++ b/gn/BUILDCONFIG.gn
@@ -27,9 +27,9 @@
 
 is_posix = !is_win
 
-# A component is either a source_set or a shared_library.
+# A component is either a static or a shared library.
 template("component") {
-  _component_mode = "source_set"
+  _component_mode = "static_library"
   if (is_component_build) {
     _component_mode = "shared_library"
   }