[devtools] Correct CSS for extension sidebar panes

Remove position relative.
Add height test to sidebar LayoutTest.

BUG=

Review URL: https://codereview.chromium.org/58933008

git-svn-id: svn://svn.chromium.org/blink/trunk@162122 bbb929c8-8fbe-4397-9dbb-9b2b20218538