Fix LayoutTests/plugins/destroy-during-npp-new.html

We crash if we try to report geometry for a plugin that has been
removed from the DOM.

This CL makes changes to main.cpp to support the layout test,
which corresponds to http://trac.webkit.org/changeset/48361.

R=jam
BUG=21841
TEST=covered by layout test

Review URL: http://codereview.chromium.org/196126

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@26234 0039d316-1c4b-4281-b951-d872f2087c98