Don't allow inline install if frame is deleted before user accepts
If the frame that called the chrome.webstore.install method to begin an
inline install gets deleted before the user accepts from the dialog, we
don't want the install to continue because a navigation could make it
look like the install request was coming from some unrelated site.
One downside of this approach is that the dialog stays around even after
the frame is deleted, and hitting either accept or cancel buttons both
just cancel the install. It would be better if the dialog is
automatically cancelled, but doing that would involve a lot more
refactoring. The approach in this CL was easier and is probably worth
getting out, and we can improve on it in the future.
Review URL: https://codereview.chromium.org/1403293008
(cherry picked from commit bbe84115d3dc969bfcf6ca87bebd1f5608db6ecf)