Early out when setting ViewportDescription if it is unchanged.
Early out when setting ViewportDescription if it is unchanged.

This causes unnecessary work, and also blocks https://codereview.chromium.org/1585793002
because it can cause a nested lifecycle run during processing of the first one
(Document::setViewportDescription can be called during style resolution).

BUG=545039

Review URL: https://codereview.chromium.org/1590123002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#369615}

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