Fix an overflow in WebCursor.
Fix an overflow in WebCursor.

I also added log message just in case a user has an issue with erroneous but unmalicious web cursor.

BUG=565023

Review URL: https://codereview.chromium.org/1498903003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#363136}

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