Remove DocumentMarker::SpellCheckClientMarkers()

This is redundant with DocumentMarker::MisspellingMarkers().

Two arguments for keeping the MisspellingMarkers() name instead of this one:

- "MisspellingMarkers" is shorter than "SpellCheckClientMarkers"

- MisspellingMarkers() has nine callsites, SpellCheckClientMarkers() only has
  one

BUG=707867

Review-Url: https://codereview.chromium.org/2836393002
Cr-Commit-Position: refs/heads/master@{#467226}
2 files changed