The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/5b95a428b6dbbcdc487b23828e5637e0b69478b1

commit 5b95a428b6dbbcdc487b23828e5637e0b69478b1
Author: droger <droger@chromium.org>
Date: Wed Jan 11 10:16:39 2017

[NoStatePrefetch] Don't cancel prefetches when there are duplicates

AutocompleteActionPredictor::StartPrerendering actively destroys the existing
prerender when a new one is created, even if the new one is a duplicate.

Thus, when there is a duplicate prerender, PrerenderManager::AddPrerender
must return a handle to it, and not null. That way it increases the
reference count on the prerender and prevents it from being destroyed.

This can be done by simply changing the ordering of the existing code.

BUG= 679714 

Review-Url: https://codereview.chromium.org/2622893002
Cr-Commit-Position: refs/heads/master@{#442844}

[modify] https://crrev.com/5b95a428b6dbbcdc487b23828e5637e0b69478b1/chrome/browser/prerender/prerender_manager.cc
[modify] https://crrev.com/5b95a428b6dbbcdc487b23828e5637e0b69478b1/chrome/browser/prerender/prerender_unittest.cc