Selection API: addRange() should be ignored if there is an existing Range.

Blink intent: https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/X48eOuLmXOE

This CL increases the number of FAIL tests in external/wpt/selection/ by 336. It's
expected, and we'll update the specification and the tests later.

* editing/selection/addRange.html and addRange-merging.html:
 They are removed because they are tests for the old behavior.

BUG=353069

Review-Url: https://codereview.chromium.org/2709443002
Cr-Commit-Position: refs/heads/master@{#451581}
35 files changed