ProxyConfigDictionary uses raw pointers to DictionaryValue to pass
ownership. This CL changes that to unique_ptr:
 * to improve code clarity
 * to go one step towards removing the obsolete raw-pointer-based
   API of base::Value, and
 * to fix a leak in ProxyConfigServiceImpl::GetActiveProxyConfigDictionary.

The problem with ProxyConfigServiceImpl::GetActiveProxyConfigDictionary was
the line:
  return base::MakeUnique<ProxyConfigDictionary>(
        ProxyConfigDictionary::CreateDirect());

In the old code, CreateDirect() would return an owning raw pointer, but
the ProxyConfigDictionary constructor would only make a Value::DeepCopy of
it and let it be, never freeing that memory.

This CL fixes the issue by switching the result of CreateDirect to be a
unique_ptr. One way to do that would to do

  return base::MakeUnique<ProxyConfigDictionary>(
        ProxyConfigDictionary::CreateDirect().get());

but that would be an unnecessary copy. Instead, this CL added a version
of the ProxyConfigDictionary constructor which accepts a unique_ptr
and avoids the copy. As a side effect, that particular line in
GetActiveProxyConfigDictionary did not have to be changed at all.

BUG=697817
TBR=bartfab@chromium.org

Review-Url: https://codereview.chromium.org/2785883003
Cr-Commit-Position: refs/heads/master@{#461076}