Fix the Fuchsia component build.

The 'http_lib' target was erroneously configured as a component rather
than a source set, which broke the component build.

Bug: 874155
Change-Id: I968b2418891d5a702f408791cd88b3077090f47d
Reviewed-on: https://chromium-review.googlesource.com/c/1290512
Commit-Queue: Fabrice de Gans-Riberi <fdegans@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601033}

webrunner/net_http/BUILD.gn

@@ -10,7 +10,7 @@ import("//build/util/process_version.gni")
 import("//testing/test.gni")
 import("//tools/grit/repack.gni")
 
-component("http_lib") {
+source_set("http_lib") {
   sources = [
     "http_service_impl.cc",
     "http_service_impl.h",