Support the DPR header for client hints.

LayoutTests/http/tests/resources/dpr.php

Index: LayoutTests/http/tests/resources/dpr.php
diff --git a/LayoutTests/http/tests/resources/dpr.php b/LayoutTests/http/tests/resources/dpr.php
new file mode 100644
index 0000000000000000000000000000000000000000..821c3dcced909d65bcf832b5d3273334a9706030
--- /dev/null
+++ b/LayoutTests/http/tests/resources/dpr.php
@@ -0,0 +1,21 @@
+<?php
+$name = $_GET['name'];
+$mimeType = $_GET['mimeType'];
+$dpr = $_GET['dpr'];
+
+header('Content-Type: ' . $mimeType);
+header('Content-Length: ' . filesize($name));
+if (isset($_GET['expires']))
+  header('Cache-control: max-age=0'); 
+else
+  header('Cache-control: max-age=86400'); 
+header('ETag: dprimage'); 
+
+if ($_SERVER['HTTP_IF_NONE_MATCH'] == 'dprimage') {
+  header('HTTP/1.1 304 Not Modified');
+  exit;
+}
+
+header('DPR: '. $dpr);
+
+readfile($name);