DevTools: The width of left navigation changes in Timeline panel when clicked on "Memory" option.

Along with the fix:
- changed sidebarSize to return 0 instead of -1
- clean up in SidebarView.

BUG=267346
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/21799002

git-svn-id: svn://svn.chromium.org/blink/trunk@155604 bbb929c8-8fbe-4397-9dbb-9b2b20218538