Gallery: use selectedIndex if leadIndex is not set.
Gallery: use selectedIndex if leadIndex is not set.

When Gallery has been re-opened, leadIndex can be -1. Use selectedIndex
as baseIndex in that case.

BUG=548121
TEST=manually tested as described in the issue.

Review URL: https://codereview.chromium.org/1430593004

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#356784}

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