Make sure hunspell file is not destroyed in UI thread

In this code:

  base::PostTaskAndReplyWithResult(
      task_runner_.get(), FROM_HERE,
      base::BindOnce(&InitializeDictionaryLocation, language_),
      base::BindOnce(&InitializeDictionaryLocationComplete,
                     weak_ptr_factory_.GetWeakPtr()));

When |SpellcheckHunspellDictionary| is destroyed before
|InitializeDictionaryLocation| is finished, the
|InitializeDictionaryLocationComplete| method would not be called, and
the returned |DictionaryFile| will be destroyed on UI thread, with its
|base::File| member.

It would then result in a DCHECK like this:

  [904:0518/140618.257:FATAL:thread_restrictions.cc(78)] Check failed: !g_blocking_disallowed.Get().Get(). Function marked as blocking was called from a scope that disallows blocking! If this task is running inside the ThreadPool, it needs to have MayBlock() in its TaskTraits. Otherwise, consider making this blocking work asynchronous or, as a last resort, you may use ScopedAllowBlocking (see its documentation for best practices).
  g_blocking_disallowed currently set to true by
  base::debug::CollectStackTrace [0x00007FF6805435C2+18] (o:\base\debug\stack_trace_win.cc:284)
  base::debug::StackTrace::StackTrace [0x00007FF68049BCC2+18] (o:\base\debug\stack_trace.cc:203)
  logging::LogMessage::~LogMessage [0x00007FF6804AF447+215] (o:\base\logging.cc:605)
  logging::LogMessage::~LogMessage [0x00007FF6804B0190+16] (o:\base\logging.cc:598)
  base::internal::AssertBlockingAllowed [0x00007FF680514B33+163] (o:\base\threading\thread_restrictions.cc:85)
  base::ScopedBlockingCall::ScopedBlockingCall [0x00007FF6805103EB+155] (o:\base\threading\scoped_blocking_call.cc:40)
  base::File::Close [0x00007FF680549E4B+139] (o:\base\files\file_win.cc:42)
  SpellcheckHunspellDictionary::DictionaryFile::~DictionaryFile [0x00007FF680071DDE+194] (o:\chrome\browser\spellchecker\spellcheck_hunspell_dictionary.cc:102)
  base::OnceCallback<void (SpellcheckHunspellDictionary::DictionaryFile)>::Run [0x00007FF6800749BF+75] (o:\base\callback.h:100)

To avoid have |base::File| destroyed on UI thread, we should make sure
the file is closed in the destructor of |DictionaryFile|.

Bug: 1085227
Change-Id: I1dba3b36661ec7968b22cf6d7835e41a81a313cc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2206199
Reviewed-by: Rouslan Solomakhin <rouslan@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Reviewed-by: Guillaume Jenkins <gujen@google.com>
Commit-Queue: Rouslan Solomakhin <rouslan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#771037}

• AUTHORS[diff]
• chrome/browser/spellchecker/spellcheck_hunspell_dictionary.cc[diff]
• chrome/browser/spellchecker/spellcheck_hunspell_dictionary.h[diff]