[oilpan]: Get rid of Document.dispose call from the Document.detach method.

With this change I have moved HTMLImportLoader and HTMLImportChild to the oilpan heap. This in combination with the previous HTMLStackItem is enough to get rid of the call to Document's dispose method under detach.

The parts that were "disposed" in the oilpan version were:

1. m_registrationContext
2. m_importsController
3. m_scripedAnimationController
4. svgExtensions()
5. m_lifeCycle.advanceTo(DocumentLifecycle::Disposed)

Ad. 1: Not clearing this refptr should not cause any leak since the RegistrationContext only has weak pointers to Element.
Ad. 2: The wasDetachedFrom cleared a cycle caused by the HTMLStackItem. It also cleared a bunch of back pointers between HTMLImportLoader and HTMLImportChild. These should no longer be needed as I have moved HTMLImportChild/Loader to the oilpan heap. Finally in HTMLImportChild the call caused the child to be removed from the parent, but only when the whole tree was destroyed. From what I can tell and from running tests it seems like it is not needed to do the remove from the parent since it is all dying.
Ad. 3 & 4. Already done in Document::detach()
Ad. 5. Not necessary AFAICT.

I ran the layout tests with the --enable-leak-detection flag. With this, and the HTMLStackItem, change we are leaking in 1985 tests. Before this change and the HTMLStackItem change we are leaking in 2076 tests.

This change also (inadvertently) fixes the below tree tests that previously failed in the oilpan build.

  fast/dom/custom/registration-context-delete-during-attribute-changed-retrieval.html [ Failure ]
  fast/dom/custom/registration-context-delete-during-register-created-retrieval.html [ Failure ]
  fast/dom/custom/registration-context-delete-then-register.html [ Failure ]

R=ager@chromium.org, erik.corry@gmail.com, haraken@chromium.org, oilpan-reviews@chromium.org, tkent@chromium.org, vegorov@chromium.org, zerny@chromium.org
BUG=

Review URL: https://codereview.chromium.org/300813002

git-svn-id: svn://svn.chromium.org/blink/trunk@175379 bbb929c8-8fbe-4397-9dbb-9b2b20218538