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This makes a specialization of base::DefaultDeleter so that scoped_ptr will
use Destroy() instead of the destructor. Without this, it seems wrong to
hold VideoDecodeAccelerator in a scoped_ptr, given that letting scoped_ptr
delete it via the destructor would be an error.

Also moves the destructors to protected, to prevent mistakes.

BUG=

Review URL: https://codereview.chromium.org/292183011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@273860 0039d316-1c4b-4281-b951-d872f2087c98