Fix windowed NPAPI plugins covering up dialogs on Win Aura.

The original fix stopped working with the new style of dialogs, since they're not parented to the WebContents anymore but instead to its parent. It also turns out we don't need to watch out for transient windows, as they're now top level ones with their own HWND so clipping works through the OS and we don't need to do anything special.

BUG=299224
R=ben@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=233297

Review URL: https://codereview.chromium.org/53153003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233398 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233398