Oilpan: Change Persistent<> data members to Member<> in HitTestResult.

- Disallow new/delete operations for HitTestResult.
  HitTestResult was used
  -- with new/delete operators
  -- as part objects of ContextMenuController and MouseEventWithHitTestResult.
  -- as stack allocated objects

  We'd like to avoid to make such classes in Oilpan.
  So, this CL disallows new/delete, and WebHitTestResult owns another
  class to hold only required data copied from a HitTestResult.

- Move ContextMenuController to Oilpan heap, and trace HitTestResult member.

- Mark MouseEventWithHitTestResults STACK_ALLOCATED.

- We can't get HitTestResult from a WebHitTestResult. So we need to
  introduce new function to WebViewImpl for WebSubstringUtil.mm.


BUG=357163

Review URL: https://codereview.chromium.org/326393003

git-svn-id: svn://svn.chromium.org/blink/trunk@176393 bbb929c8-8fbe-4397-9dbb-9b2b20218538