Null-check ImageBitmap's SkImage in texImage2D

This should guard against a crash.
It's not clear exactly what the cause of this null is, but it's
probably some kind of failure to allocate the SkImage (e.g.
out-of-memory, GPU process loss, or other unpredictable failure case).

Bug: 852303
Change-Id: I10a817f2dccd801f5834be343ad687af09e06721
Reviewed-on: https://chromium-review.googlesource.com/1188515Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Kai Ninomiya <kainino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#586085}

third_party/blink/renderer/modules/webgl/webgl_rendering_context_base.cc

@@ -5594,8 +5594,15 @@ void WebGLRenderingContextBase::TexImageHelperImageBitmap(
     }
     return;
   }
+
   sk_sp<SkImage> sk_image =
       bitmap->BitmapImage()->PaintImageForCurrentFrame().GetSkImage();
+  if (!sk_image) {
+    SynthesizeGLError(GL_OUT_OF_MEMORY, func_name,
+                      "ImageBitmap unexpectedly empty");
+    return;
+  }
+
   SkPixmap pixmap;
   uint8_t* pixel_data_ptr = nullptr;
   scoped_refptr<Uint8Array> pixel_data;