No direct assignment on `src` attribute

Change-Id: Iff7a0fa0f6ae880d3fabd4ad8d9eefe96eebbcad

gr-resemble-diff-mode/gr-resemble-diff-mode.js

diff --git a/gr-resemble-diff-mode/gr-resemble-diff-mode.js b/gr-resemble-diff-mode/gr-resemble-diff-mode.js
index f7dec89..c7b7cba 100644
--- a/gr-resemble-diff-mode/gr-resemble-diff-mode.js
+++ b/gr-resemble-diff-mode/gr-resemble-diff-mode.js
@@ -76,8 +76,8 @@
   }
 
   _setImageDiffSrc(src) {
-    delete this.$.imageDiff.src;
-    this.$.imageDiff.src = src;
+    this.$.imageDiff.removeAttribute('src');
+    this.$.imageDiff.setAttribute('src', src);
   }
 
   _setDifferenceValue(percentage) {