Fixing issue dereferencing skImage

When we transferToImageBitmap and we do a flush in the Snapshot, it can
be that the skImage was null due to some issue.

We are adding a new check to ensure that we are not returning a null
skImage contained in a valid imagebitmap.

Bug: 980057
Change-Id: I91e6b1688ec54b6029bcdd051db3d045421786cc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1811157Reviewed-by: Fernando Serboncini <fserb@chromium.org>
Commit-Queue: Juanmi Huertas <juanmihd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#697787}

third_party/blink/renderer/modules/canvas/offscreencanvas2d/offscreen_canvas_rendering_context_2d.cc

@@ -182,8 +182,13 @@ ImageBitmap* OffscreenCanvasRenderingContext2D::TransferToImageBitmap(
   if (image->IsTextureBacked()) {
     // Before discarding the image resource, we need to flush pending render ops
     // to fully resolve the snapshot.
-    image->PaintImageForCurrentFrame().GetSkImage()->getBackendTexture(
-        true);  // Flush pending ops.
+    // We can only do this if the skImage is not null
+    if (auto skImage = image->PaintImageForCurrentFrame().GetSkImage()) {
+      skImage->getBackendTexture(true);  // Flush pending ops.
+    } else {
+      // If the SkImage was null, we better return a null ImageBitmap
+      return nullptr;
+    }
   }
   Host()->DiscardResourceProvider();  // "Transfer" means no retained buffer.
   return ImageBitmap::Create(std::move(image));