Add checking for runtime errors in the backgroud page in the Files App.

Since the background page is getting more and more complex, we should check for errors as we do it for windows. This patch adds such counter, and modifies the test utils responsible for summing the errors.

TEST=browser_tests
BUG=299503

Review URL: https://codereview.chromium.org/25674011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226737 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226737