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Adding split view divider widget which could be dragged to exit the split view.
Resubmit of https://codereview.chromium.org/545393002

TBR=mukai@chromium.org
BUG=403207, 408691

Review URL: https://codereview.chromium.org/598083002

Cr-Commit-Position: refs/heads/master@{#296464}