Rename SharedEventContext to TreeScopeEventContext after r164501.

This is a follow-up patch for r164501, where I forgot to rename some variable and function names.

BUG=329445

Review URL: https://codereview.chromium.org/125033002

git-svn-id: svn://svn.chromium.org/blink/trunk@164560 bbb929c8-8fbe-4397-9dbb-9b2b20218538