DevTools: Fix issues resulting from async panel loading and IFH.loadCompleted()

- Ctrl+Shift+C (open in Inspect Element mode) did not allow to actually select
  an element in the inspected page.
- Ctrl+Shift+J (open Console panel) sometimes did not open the console panel
  but the last panel instead.

TBR=pfeldman@chromium.org
BUG=421369

Review URL: https://codereview.chromium.org/635263002

git-svn-id: svn://svn.chromium.org/blink/trunk@183406 bbb929c8-8fbe-4397-9dbb-9b2b20218538