Add a missing null check for WebElement::ImageContents()

Bug: 869581
Change-Id: Ibe28c2ed55441c74d9faa845f81801c762559de7
Reviewed-on: https://chromium-review.googlesource.com/1184132
Commit-Queue: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585379}

third_party/blink/renderer/core/exported/web_element.cc

@@ -155,9 +155,10 @@ WebRect WebElement::BoundsInViewport() const {
 SkBitmap WebElement::ImageContents() {
   if (IsNull())
     return {};
-
-  return Unwrap<Element>()->ImageContents()->AsSkBitmapForCurrentFrame(
-      kRespectImageOrientation);
+  Image* image = Unwrap<Element>()->ImageContents();
+  if (!image)
+    return {};
+  return image->AsSkBitmapForCurrentFrame(kRespectImageOrientation);
 }
 
 void WebElement::RequestFullscreen() {