DevTools: NetworkPanel: Preview: do not try to show HTML as JSONP.

Source/devtools/front_end/network/RequestPreviewView.js

Index: Source/devtools/front_end/network/RequestPreviewView.js
diff --git a/Source/devtools/front_end/network/RequestPreviewView.js b/Source/devtools/front_end/network/RequestPreviewView.js
index be57b3e7098551fcca3925b3f998a2629247f8e0..da6e8a8b4422eab4ec714e5207ec77595afcebbd 100644
--- a/Source/devtools/front_end/network/RequestPreviewView.js
+++ b/Source/devtools/front_end/network/RequestPreviewView.js
@@ -144,12 +144,20 @@ WebInspector.RequestPreviewView.prototype = {
             return xmlView;
 
         if (this.request.resourceType() === WebInspector.resourceTypes.XHR) {
-            var jsonView = this._jsonView();
-            if (jsonView)
-                return jsonView;
-            var htmlErrorPreview = this._htmlErrorPreview();
-            if (htmlErrorPreview)
-                return htmlErrorPreview;
+            var isHtml = false;
+            try {
+                isHtml = !!((new DOMParser()).parseFromString(this._requestContent(), mimeType)).body;
+            } catch (e) {
+            }
+            if (!isHtml) {
+                var jsonView = this._jsonView();
+                if (jsonView)
+                    return jsonView;
+            } else {
+                var htmlErrorPreview = this._htmlErrorPreview();
+                if (htmlErrorPreview)
+                    return htmlErrorPreview;
+            }
         }
 
         if (this._responseView.sourceView)