DevTools: Do not save split view size on show mode changes.

BUG=348887
R=dgozman@chromium.org, dgozman

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=168608

Review URL: https://codereview.chromium.org/185943004

git-svn-id: svn://svn.chromium.org/blink/trunk@168775 bbb929c8-8fbe-4397-9dbb-9b2b20218538