[HeapMojo] Fix WebSettingsTest#testAccessCacheMode failure on Android L and M

Still not sure the root cause, but WebSettingsTest#testAccessCacheMode is failing on Android L and M. And this failure is caused by HeapMojoReceiver for ImageDownloaderImpl.receiver_.

This CL specifies HeapMojoWrapperMode::kForceWithoutContextObserver not to close connection when the associated ExecutionContext is detached.

Bug: 1102041, 1049056
Change-Id: I4a88ec57f2b04671a2d7ffad953600713442f648
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2395195Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Minoru Chikamune <chikamune@chromium.org>
Cr-Commit-Position: refs/heads/master@{#804666}

third_party/blink/renderer/modules/image_downloader/image_downloader_impl.h

@@ -94,7 +94,7 @@ class ImageDownloaderImpl final : public GarbageCollected<ImageDownloaderImpl>,
 
   HeapMojoReceiver<mojom::blink::ImageDownloader,
                    ImageDownloaderImpl,
-                   HeapMojoWrapperMode::kWithoutContextObserver>
+                   HeapMojoWrapperMode::kForceWithoutContextObserver>
       receiver_;
 
   DISALLOW_COPY_AND_ASSIGN(ImageDownloaderImpl);