Replace TaskType::kUnthrottled with kInternalLoading where loading is related

This changes a behavior slightly: the task runner is change from
unthrottleable to throttleable. However, there should not be a strong
reason why the task runner should be unthrottled there.

We plan to eliminate kUnthrottled, kUnspecedTimer and kUnspecedLoading
and replace them with appropriate kInternal* task types.

Task type guideline: https://bit.ly/2vMAsQ4

Bug: 830489
Change-Id: I55519cb83aa40cbf4fd98bcd18a9424aa9a7b882
Reviewed-on: https://chromium-review.googlesource.com/1051308
Commit-Queue: Hajime Hoshi <hajimehoshi@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557165}

content/renderer/image_downloader/image_downloader_base.cc

@@ -111,7 +111,7 @@ void ImageDownloaderBase::DidFetchImage(
       iter->release();
       image_fetchers_.erase(iter);
       render_frame()
-          ->GetTaskRunner(blink::TaskType::kUnthrottled)
+          ->GetTaskRunner(blink::TaskType::kInternalLoading)
           ->DeleteSoon(FROM_HERE, fetcher);
       break;
     }