Correct self assignment in LayoutRectPosition.
Correct self assignment in LayoutRectPosition.

This was introduced in https://codereview.chromium.org/1312763007/ and caught
by the static-analyzer.

BUG=

Review URL: https://codereview.chromium.org/1712963002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#376422}

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