DevTools: [SSP] simplify sidebar panes update

The patch removes woeful "needsUpdate" flags that were set by
ElementsPanel on its sidebar panes.

Now, "update" methods of all sidebars in ElementsPanel postpone work to
update rendering until it becomes visible.

BUG=427396
R=apavlov, pfeldman

Review URL: https://codereview.chromium.org/688613003

git-svn-id: svn://svn.chromium.org/blink/trunk@184744 bbb929c8-8fbe-4397-9dbb-9b2b20218538