Make chrome GN build work in component mode.

content/public/child/BUILD.gn

Index: content/public/child/BUILD.gn
diff --git a/content/public/child/BUILD.gn b/content/public/child/BUILD.gn
index 9a7c49e5d9c9d1c45b50cb3422506df4d756a17d..1d2077da65f529eb244e134d2c37d69bef853cbd 100644
--- a/content/public/child/BUILD.gn
+++ b/content/public/child/BUILD.gn
@@ -4,7 +4,19 @@
 
 import("//content/child/child.gni")
 
-source_set("child") {
+# See //content/BUILD.gn for how this works.
+group("child") {
+  if (is_component_build) {
+    deps = [ "//content" ]
+  } else {
+    deps = [ ":child_sources" ]
+  }
+  forward_dependent_configs_from = deps
+}
+
+source_set("child_sources") {
+  visibility = [ "//content/*" ]
+
   sources = rebase_path(content_child_gypi_values.public_child_sources,
                         ".", "//content")
 
@@ -14,6 +26,6 @@ source_set("child") {
 
   deps = [
     "//content/child",
-    "//content/public/common",
+    "//content/public/common:common_sources",
   ]
 }