DevTools: _base64ToSize doesn't check content param for null.

BUG=425514

Review URL: https://codereview.chromium.org/672573002

git-svn-id: svn://svn.chromium.org/blink/trunk@184170 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/devtools/front_end/source_frame/ImageView.js

@@ -93,9 +93,13 @@ WebInspector.ImageView.prototype = {
         this._imagePreviewElement = imagePreviewElement;
     },
 
+    /**
+     * @param {?string} content
+     * @return {number}
+     */
     _base64ToSize: function(content)
     {
-        if (!content.length)
+        if (!content || !content.length)
             return 0;
         var size = (content.length || 0) * 3 / 4;
         if (content.length > 0 && content[content.length - 1] === "=")