Introduce background.scripts feature for extension manifests.

This optimizes for the common use case where background pages
just include a reference to one or more script files and no
additional HTML.

BUG=107791


Review URL: http://codereview.chromium.org/9150008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@117110 0039d316-1c4b-4281-b951-d872f2087c98
void BackgroundContentsService::BackgroundContentsShutdown(
    BackgroundContents* background_contents) {
  DCHECK(IsTracked(background_contents));
  string16 appid = GetParentApplicationId(background_contents);
  contents_map_.erase(appid);
}

void BackgroundContentsService::BackgroundContentsShutdown(
    BackgroundContents* background_contents) {
  DCHECK(IsTracked(background_contents));
  string16 appid = GetParentApplicationId(background_contents);
  contents_map_.erase(appid);
}
