[DevTools] Preserve screen cast sidebar width.
[DevTools] Preserve screen cast sidebar width.

BUG=307603

Review URL: https://codereview.chromium.org/27718003

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@159912 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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