Avoid unnecessary allocation in EventPath::ensureTreeScopeEventContext.

We always allocated new TreeScopeEventContext object for a TreeScope even if
another object for the TreeScope is in TreeScopeEventContextMap. We should apply
an idiom of HashMap::AddResult that we pass nullptr to HashMap::add and check if
AddResult.isNewEntry.

BUG=364209

Review URL: https://codereview.chromium.org/246303006

git-svn-id: svn://svn.chromium.org/blink/trunk@172335 bbb929c8-8fbe-4397-9dbb-9b2b20218538