Fix chrome.runtime.sendMessage background page memory leak.

BUG=320723
R=jyasskin@chromium.org

Review URL: https://codereview.chromium.org/83023014

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236923 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236923