Include "public" source files in generated Xcode project.

Targets can restrict the list of public headers by using "public"
property in addition to "sources". Include them in the generated
Xcode project as any other source file.

BUG=None

Review-Url: https://codereview.chromium.org/2740123003
Cr-Commit-Position: refs/heads/master@{#456339}

tools/gn/xcode_writer.cc

@@ -404,7 +404,7 @@ void XcodeWriter::CreateProductsProject(
       new PBXProject("products", config_name, source_path, attributes));
   SourceDir source_dir("//");
 
-  // Add all source files for indexing.
+  // Add all source files for indexing, both private and public.
   std::vector<SourceFile> sources;
   for (const Target* target : all_targets) {
     for (const SourceFile& source : target->sources()) {
@@ -413,6 +413,16 @@ void XcodeWriter::CreateProductsProject(
 
       sources.push_back(source);
     }
+
+    if (target->all_headers_public())
+      continue;
+
+    for (const SourceFile& source : target->public_headers()) {
+      if (IsStringInOutputDir(build_settings->build_dir(), source.value()))
+        continue;
+
+      sources.push_back(source);
+    }
   }
 
   // Sort sources to ensure determinisn of the project file generation and