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If a DNS probe happens before the previous probe completes,
the previous callback will never be called, leading to a
JS stack leak.  This change resolves the first callback
to free up the stack.

This leak is unlikely in practice.  If a leak does occur,
it persists until the user closes the settings page.

Change-Id: Id12a4181bdbe1274c7441d9be9bf223d2a457406
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2132589
Commit-Queue: Ben Schwartz <bemasc@chromium.org>
Auto-Submit: Ben Schwartz <bemasc@chromium.org>
Reviewed-by: Esmael Elmoslimany <aee@chromium.org>
Reviewed-by: Eric Orth <ericorth@chromium.org>
Cr-Commit-Position: refs/heads/master@{#758550}