PDF: Use the z-order when there is a link and widget overlaps.
PDF: Use the z-order when there is a link and widget overlaps.

BUG=515837

Review URL: https://codereview.chromium.org/1274413002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#343574}

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