Prevent calling MakeThreadSafe on null ImageSkia

It is unclear why the CHECK in MakeThreadSafe is getting hit, but this
should prevent that. This case should be caught by the existing
if (!optional_image) checks, but apparently is not.

Bug: 1136759
Change-Id: Ia38f25df99c0f39fcdc002bb78ec447fdc4e58cf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2484701
Commit-Queue: Robbie Gibson <rkgibson@google.com>
Reviewed-by: Justin Donnelly <jdonnelly@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818504}

components/omnibox/browser/clipboard_provider.cc

@@ -552,7 +552,10 @@ void ClipboardProvider::NewClipboardImageMatch(
 void ClipboardProvider::OnReceiveImage(
     ClipboardImageMatchCallback callback,
     base::Optional<gfx::Image> optional_image) {
-  if (!optional_image) {
+  // ImageSkia::ToImageSkia should only be called if the gfx::Image is
+  // non-empty. It is unclear when the clipboard returns a non-optional but
+  // empty image. See crbug.com/1136759 for more details.
+  if (!optional_image || optional_image.value().IsEmpty()) {
     std::move(callback).Run(base::nullopt);
     return;
   }