Gn: Add support to generate Visual Studio 2017 solutions.

tools/gn/visual_studio_writer.cc

Index: tools/gn/visual_studio_writer.cc
diff --git a/tools/gn/visual_studio_writer.cc b/tools/gn/visual_studio_writer.cc
index 6e0b1bd835d8f61a6810defc938e7da6dea642aa..8a8cf528fc15c5ae99b1081a62b187f3c03d5ae7 100644
--- a/tools/gn/visual_studio_writer.cc
+++ b/tools/gn/visual_studio_writer.cc
@@ -71,10 +71,13 @@ struct SourceFileWriter {
 
 const char kToolsetVersionVs2013[] = "v120";               // Visual Studio 2013
 const char kToolsetVersionVs2015[] = "v140";               // Visual Studio 2015
+const char kToolsetVersionVs2017[] = "v141";               // Visual Studio 2017
 const char kProjectVersionVs2013[] = "12.0";               // Visual Studio 2013
 const char kProjectVersionVs2015[] = "14.0";               // Visual Studio 2015
+const char kProjectVersionVs2017[] = "15.0";               // Visual Studio 2015
 const char kVersionStringVs2013[] = "Visual Studio 2013";  // Visual Studio 2013
 const char kVersionStringVs2015[] = "Visual Studio 2015";  // Visual Studio 2015
+const char kVersionStringVs2017[] = "Visual Studio 2017";  // Visual Studio 2017
 const char kWindowsKitsVersion[] = "10";                   // Windows 10 SDK
 const char kWindowsKitsIncludeVersion[] = "10.0.14393.0";  // Windows 10 SDK
 
@@ -273,6 +276,11 @@ VisualStudioWriter::VisualStudioWriter(const BuildSettings* build_settings,
       toolset_version_ = kToolsetVersionVs2015;
       version_string_ = kVersionStringVs2015;
       break;
+    case Version::Vs2017:
+      project_version_ = kProjectVersionVs2017;
+      toolset_version_ = kToolsetVersionVs2017;
+      version_string_ = kVersionStringVs2017;
+      break;
     default:
       NOTREACHED() << "Not a valid Visual Studio Version: " << version;
   }