Add default_library_type (flutter#188)

Some fuchsia deps are depending on this upstream.

Author: goderbauer

build/config/BUILDCONFIG.gn

@@ -271,6 +271,14 @@ if (current_os == "win") {
   is_win = false
 }
 
+# Some library targets may be built as different type depending on the target
+# platform. This variable specifies the default library type for each target.
+if (is_fuchsia) {
+  default_library_type = "shared_library"
+} else {
+  default_library_type = "static_library"
+}
+
 # =============================================================================
 # SOURCES FILTERS
 # =============================================================================