GitLab

This CL is fixing the memory leak describe here:
  https://bugs.chromium.org/p/chromium/issues/detail?id=974239

Without this patch, tasks to check the file existence can be
queued for the same download without being executed. This can
lead to terrible scenario with billion of tasks queued.

This CL is adding a set to keep track of the current pending
download queries. This way, there can not be two pending tasks
at the same time for each download item.

Bug: 974239
Change-Id: I52fb23ce50ba8542ed262552dd7f9aaaa5589548
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1946783Reviewed-by: François Doray <fdoray@chromium.org>
Reviewed-by: Min Qin <qinmin@chromium.org>
Commit-Queue: Etienne Bergeron <etienneb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#720600}