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This matches other webui dialogs' (settings) behavior by setting initial
focus when a dialog is shown.

To fully match we need to also restore focus when the dialog is closed,
which we'll do when it's easier (i.e. when the page isn't totally
rebuilt all the time).

BUG=438301
R=kalman@chromium.org

Review URL: https://codereview.chromium.org/900623003

Cr-Commit-Position: refs/heads/master@{#314629}