Cleanup in handling of JavaScript URLs in tabs API

      Reuse infrastructure for chrome.tabs.executeScript() to
implement chrome.tabs.update({url:&quotjavascript:....&quot}) since
it has built-in safeguards against navigation races.

BUG=77026

Review URL: http://codereview.chromium.org/6771062

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@80826 0039d316-1c4b-4281-b951-d872f2087c98