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This patch gets rid of flat tree version of |createVisibleSelection()| taking
two |VisiblePosition| by replacing with |SelectionInDOMTree| version to reduce
number of overloads for improving code health.

BUG=657237
TEST=n/a; no behavior changes

Review-Url: https://codereview.chromium.org/2437873008
Cr-Commit-Position: refs/heads/master@{#427031}