DevTools: complete removal of InspectorView._panel and InspectorView._showPanel.

This change pretty much removes those calls from the tests.

TBR=apavlov
NOTRY=true

Review URL: https://codereview.chromium.org/617173002

git-svn-id: svn://svn.chromium.org/blink/trunk@183032 bbb929c8-8fbe-4397-9dbb-9b2b20218538