Add some tests for DocumentMarkerController::removeMarkers()

I'm changing the behavior of removeMarkers() in https://codereview.chromium.org/2806683002 to always remove markers rather than splitting and noticed that the current behavior doesn't seem to be covered by any tests; I'm adding tests for the current behavior to make it clear in that CL how the behavior is being changed.

BUG=707867

Review-Url: https://codereview.chromium.org/2812723002
Cr-Commit-Position: refs/heads/master@{#463899}
1 file changed