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.

Difference from r176393:
Always calling HitTestResult member functions was bad. Some of them are
expensive. So WebHitTestResultPrivate contains a HitTestResult copy, and
WebHitTestResultPrivate is RefCountedWillBeGarbageCollectedFinalized in order to
trace HitTestResult.

BUG=357163

Review URL: https://codereview.chromium.org/413903004

git-svn-id: svn://svn.chromium.org/blink/trunk@178997 bbb929c8-8fbe-4397-9dbb-9b2b20218538