Added checks to prune out redundant download updates

Download file keeps sending us updates about its status in a loop even
though the download can be in a paused state. Added a check to prune out
these updates if there is no change to the bytes received. This will
enable us to clean up the code to avoid the state-dependent
AddObserver/RemoveObserver calls in the download observers such as
DownloadController.

Bug: 913085
Change-Id: I915de9d8f95ba5320529ddfe761404c008678576
Reviewed-on: https://chromium-review.googlesource.com/c/1368650Reviewed-by: Min Qin <qinmin@chromium.org>
Commit-Queue: Shakti Sahu <shaktisahu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615361}

chrome/browser/android/download/download_controller.cc

@@ -301,6 +301,7 @@ void DownloadController::CreateAndroidDownload(
 }
 
 void DownloadController::AboutToResumeDownload(DownloadItem* download_item) {
+  download_item->RemoveObserver(this);
   download_item->AddObserver(this);
 
   // If a download is resumed from an interrupted state, record its strong
@@ -411,8 +412,6 @@ void DownloadController::OnDownloadUpdated(DownloadItem* item) {
   switch (item->GetState()) {
     case DownloadItem::IN_PROGRESS: {
       Java_DownloadController_onDownloadUpdated(env, j_item);
-      if (item->IsPaused())
-        item->RemoveObserver(this);
       break;
     }
     case DownloadItem::COMPLETE:
@@ -438,7 +437,6 @@ void DownloadController::OnDownloadUpdated(DownloadItem* item) {
       // resume in this case.
       Java_DownloadController_onDownloadInterrupted(env, j_item,
           IsInterruptedDownloadAutoResumable(item));
-      item->RemoveObserver(this);
       break;
     case DownloadItem::MAX_DOWNLOAD_STATE:
       NOTREACHED();

components/download/internal/common/download_item_impl.cc

@@ -1234,6 +1234,9 @@ void DownloadItemImpl::DestinationUpdate(
                        TRACE_EVENT_SCOPE_THREAD, "bytes_so_far",
                        GetReceivedBytes());
 
+  if (IsPaused() && destination_info_.received_bytes == bytes_so_far)
+    return;
+
   UpdateObservers();
 }