Null-check RenderView in PepperPluginInstance's destructor.

BUG=335940
R=bbudge@chromium.org

Review URL: https://codereview.chromium.org/132333009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247277 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247277