[iOS] Creating signin_headers and signin_impl target in authentication/signin

authentication/signin:signin is the public target to depend on.
authentication/signin:signin_headers contains the public headers.
authentication/signin:signin_impl contains the sign-in implementation.

Related to: crrev.com/c/2025547

Bug: 971989
Change-Id: If7cbaf76df94b4a2647d0f47f56912cd3f36d0de
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2027809
Commit-Queue: Jérôme Lebel <jlebel@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#736510}

ios/chrome/browser/ui/authentication/signin/BUILD.gn

@@ -4,7 +4,14 @@
 
 import("//build/config/chrome_build.gni")
 
-source_set("signin") {
+# Public target.
+group("signin") {
+  public_deps = [ ":signin_headers" ]
+  deps = [ ":signin_impl" ]
+}
+
+# Contains public headers.
+source_set("signin_headers") {
   configs += [ "//build/config/compiler:enable_arc" ]
   sources = [ "signin_coordinator.h" ]
   public_deps = [
@@ -14,8 +21,9 @@ source_set("signin") {
   ]
 }
 
-source_set("private") {
+# Contains sign-in implementation.
+source_set("signin_impl") {
   configs += [ "//build/config/compiler:enable_arc" ]
   sources = [ "signin_coordinator.mm" ]
-  deps = [ ":signin" ]
+  deps = [ ":signin_headers" ]
 }