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This removes an aura dependency in webview/web_dialog_view.cc and is a
step towards getting the views_examples target compiling on Mac.

NativeWidget::GetEventHandler isn't used elsewhere.

Pre-Aura Win would use ::DefWindowProc() in
WebDialogView::HandleKeyboardEvent(), so NativeWidget feels like the
right place for this.

BUG=366007
TEST=Just refactoring

Review URL: https://codereview.chromium.org/307623002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@273793 0039d316-1c4b-4281-b951-d872f2087c98