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This size is used to determine when autosize has resized the guest.

BUG=273089
Test=Launch chrome with --enable-embedded-extension-options, Open up chrome://extensions, click on Options for Goolge SPDY Proxy VPN and it shows a scrollbar.

Review URL: https://codereview.chromium.org/544313002

Cr-Commit-Position: refs/heads/master@{#293611}