[content shell] if the document is plain text, dump it as text

content/shell/webkit_test_controller.cc

Index: content/shell/webkit_test_controller.cc
diff --git a/content/shell/webkit_test_controller.cc b/content/shell/webkit_test_controller.cc
index 858e0d968c3d33c2608ad9d89b29f43504145e79..7180b08ff2e4f0eee3936ca3c6b4760b615c5c0b 100644
--- a/content/shell/webkit_test_controller.cc
+++ b/content/shell/webkit_test_controller.cc
@@ -274,6 +274,11 @@ void WebKitTestController::CaptureDump() {
     return;
   captured_dump_ = true;
 
+  if (main_window_->web_contents()->GetContentsMimeType() == "text/plain") {
+    dump_as_text_ = true;
+    enable_pixel_dumping_ = false;
+  }
+
   RenderViewHost* render_view_host =
       main_window_->web_contents()->GetRenderViewHost();