Revert "[GN] Remove a duplication of a source type check"

This reverts commit 844a031c.

Reason for revert: This CL causes failures when generating Xcode project on iOS platform, please see comments inline.

Original change's description:
> [GN] Remove a duplication of a source type check
> 
> There was a duplication of the source type checking. It may lead
> errors when a type set will be changed for a language.
> 
> R=​dpranke@chromium.org
> 
> Bug: None
> Change-Id: Ieab8885899446e625cfe20c1650418a085b937ee
> Reviewed-on: https://chromium-review.googlesource.com/702414
> Reviewed-by: Dirk Pranke <dpranke@chromium.org>
> Commit-Queue: Dirk Pranke <dpranke@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#506763}

TBR=dpranke@chromium.org,vladbelov@yandex-team.ru

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: None
Change-Id: I5dd0a68f9dd9ac1edd6b94e248bdf5959f83b3ab
Reviewed-on: https://chromium-review.googlesource.com/747087Reviewed-by: Yuke Liao <liaoyuke@chromium.org>
Commit-Queue: Yuke Liao <liaoyuke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512888}

tools/gn/xcode_object.cc

@@ -13,8 +13,6 @@
 #include "base/memory/ptr_util.h"
 #include "base/strings/string_util.h"
 #include "tools/gn/filesystem_utils.h"
-#include "tools/gn/source_file.h"
-#include "tools/gn/source_file_type.h"
 
 // Helper methods -------------------------------------------------------------
 
@@ -155,10 +153,9 @@ bool HasExplicitFileType(const base::StringPiece& ext) {
   return ext == "dart";
 }
 
-bool IsSourceFileForIndexing(const SourceFile& src) {
-  const SourceFileType type = GetSourceFileType(src);
-  return type == SOURCE_C || type == SOURCE_CPP || type == SOURCE_M ||
-         type == SOURCE_MM;
+bool IsSourceFileForIndexing(const base::StringPiece& ext) {
+  return ext == "c" || ext == "cc" || ext == "cpp" || ext == "cxx" ||
+         ext == "m" || ext == "mm";
 }
 
 void PrintValue(std::ostream& out, IndentRules rules, unsigned value) {
@@ -695,7 +692,8 @@ void PBXProject::AddSourceFile(const std::string& navigator_path,
                                PBXNativeTarget* target) {
   PBXFileReference* file_reference =
       sources_->AddSourceFile(navigator_path, source_path);
-  if (!IsSourceFileForIndexing(SourceFile(source_path)))
+  base::StringPiece ext = FindExtension(&source_path);
+  if (!IsSourceFileForIndexing(ext))
     return;
 
   DCHECK(target);