Fix chrome.webRequest.onBeforeRequest for requests coming from NPAPI plugin processes.

BUG=332481
R=jochen@chromium.org

Review URL: https://codereview.chromium.org/132233022

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245847 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245847