Clean up DocumentMarkerController::MarkersFor()

I'm cleaning up two things here:

- Adding a check to PossiblyHasMarkers() (probably doesn't save us very much
  time, but we can at least avoid a hashmap lookup in the "no markers" case)
- Fixing the loop to just loop over the passed-in set of MarkerTypes vs. looping
  over all types and skipping over some of them

BUG=707867

Review-Url: https://codereview.chromium.org/2956453002
Cr-Commit-Position: refs/heads/master@{#485038}
1 file changed