Pepper: Fix renderer crash on plugin destruction.

Pepper: Fix renderer crash on plugin destruction.

The FileDownloader refactor caused callbacks to be more tightly bound to
PnaclCoordinator than they were previously. Before the refactor, callbacks that
were invoked as the pexe was downloaded (or the cached translated nexe was
received) were generated through the CompletionCallbackFactory interface, which
would cause them to be cancelled when PnaclCoordinator was destroyed.

This change checks that the plugin instance is still alive before calling any
of the callbacks in the PPP_PexeStreamHandler interface.

I tried conducting some local testing, but didn't manage to hit quite the same
codepath as the one reported in the bug.

BUG=400171

Review URL: https://codereview.chromium.org/433633003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@287472 0039d316-1c4b-4281-b951-d872f2087c98