Speculative fix for Use-after-free in FetchManager.
Speculative fix for Use-after-free in FetchManager.

This CL is a speculative fix for a use-after-free bug in FetchManager. My
scenario is here:

1. FetchManager::onLoaderFinished removes a loader from |m_loaders| and that
   deletes the loader.
2. In the destructor of the loader, it cancels loading.
3. The cancel operation calls FetchManager::onLoaderFinished again.
4. The removing operation is not yet done at this point and thus the destructor
   for the same loader is called again.

BUG=441095, 441138

Review URL: https://codereview.chromium.org/795003002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@186939 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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