Allow image documents in MHT if there is only one resource.

third_party/WebKit/LayoutTests/mhtml/image_document.mht

Index: third_party/WebKit/LayoutTests/mhtml/image_document.mht
diff --git a/third_party/WebKit/LayoutTests/mhtml/image_document.mht b/third_party/WebKit/LayoutTests/mhtml/image_document.mht
new file mode 100644
index 0000000000000000000000000000000000000000..6c97fb5961dd0f6985fcd2815a499440b02837ec
--- /dev/null
+++ b/third_party/WebKit/LayoutTests/mhtml/image_document.mht
@@ -0,0 +1,19 @@
+From: <Saved by Blink>
+Subject: 128x128_png.png (128?128)
+Date: Fri, 13 Oct 2016 17:40:08 -0000
+MIME-Version: 1.0
+Content-Type: multipart/related;
+	type="text/html";
+	boundary="----MultipartBoundary--Xm93PP2BFJf8elJGRutH4cfnMaBMcK4sxf4o3u4L2F----"
+
+------MultipartBoundary--Xm93PP2BFJf8elJGRutH4cfnMaBMcK4sxf4o3u4L2F----
+Content-Type: image/png
+Content-Transfer-Encoding: base64
+Content-Location: http://localhost/resources/red_square.png
+
+iVBORw0KGgoAAAANSUhEUgAAAB4AAAAeCAYAAAA7MK6iAAAAAXNSR0IArs4c6QAAAARnQU1BAACx
+jwv8YQUAAAAJcEhZcwAADsQAAA7EAZUrDhsAAACESURBVEhL7ZYxDoAwCEXB2LGTo46ewXj/zXgV
+PQKgexP/Ai6l889/6Stp4HtZjYKOiRCpEpfSEIYgJqxNMFTkFUjVXiZhT6qGirwCqdrLJOxJ1VCR
+V6A/1XxNc9jqQ6JkpjTU2rwQR+5c4769YCM5zn/BX4PY33Dljb0+JtiTqqEir8ADWEIY6dz7d1cA
+AAAASUVORK5CYII=
+------MultipartBoundary--Xm93PP2BFJf8elJGRutH4cfnMaBMcK4sxf4o3u4L2F------