GN: Fix single-file compilations in VS2017 projects

Setting the "WindowsTargetPlatformVersion" XML element seems to
be necessary in VS2017. Single-file compilations do not work
without it. It does not cause any problems in older VS versions.

R=brettw@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2858273002
Cr-Commit-Position: refs/heads/master@{#472272}

tools/gn/visual_studio_writer.cc

@@ -435,6 +435,8 @@ bool VisualStudioWriter::WriteProjectFileContents(
     globals->SubElement("RootNamespace")->Text(target->label().name());
     globals->SubElement("IgnoreWarnCompileDuplicatedFilename")->Text("true");
     globals->SubElement("PreferredToolArchitecture")->Text("x64");
+    globals->SubElement("WindowsTargetPlatformVersion")
+        ->Text(kWindowsKitsIncludeVersion);
   }
 
   project.SubElement(