Remove bad DCHECK. I was hitting this every time an image had an error.

TBR=nektar@chromium.org

Bug: 905419
Change-Id: Iabcb0eff62369972c07aef952fa6345422b761ee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1507256Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638484}

content/renderer/accessibility/ax_image_annotator.h

@@ -70,7 +70,6 @@ class CONTENT_EXPORT AXImageAnnotator : public base::CheckedObserver {
     }
 
     std::string annotation() const {
-      DCHECK(annotation_.has_value());
       return annotation_.value_or("");
     }