drive: Support authorizing third-party Drive apps for opening files.

Along the way, replace the official key check in DriveAppRegistry::IsUninstallSupported
by the new devoted function. (In fact, the macro is available
only during google_apis/ hence the previous code was not working well...)


BUG=332332

Review URL: https://codereview.chromium.org/140783006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245565 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245565