Change DMC::AddCompositionMarker() to take EphemeralRange

Change DocumentMarkerController::AddCompositionMarker() from taking two
Positions (start and end) to taking an EphemeralRange. This matches
AddTextMatchMarker() and is how we eventually want all the Add*Marker() methods
to work.

BUG=707867

Review-Url: https://codereview.chromium.org/2899923002
Cr-Commit-Position: refs/heads/master@{#474480}
5 files changed