Make DragController::DragIsMove return false when selection does not have focus

This patch makes the above change so that when dragging with an unfocused
selection, the selection is not considered as the dragging source.

BUG=717453
TEST=editing/selection/drag_with_unfocused_selection.html

Review-Url: https://codereview.chromium.org/2877623005
Cr-Commit-Position: refs/heads/master@{#471253}
2 files changed