https://bugs.webkit.org/show_bug.cgi?id=13631

Reviewed by Sam Weinig.

This patch is primarily about the "CachedFrame tree" and making slightly different decisions
based on whether a CachedFrame is the main frame or a subframe.

- Store whether a CachedFrame represents the main frame or a subframe.
- "Chop off the frame tree" for the main frame when creating its CachedFrame, as main frames 
  are reused and need to start new page loads with an empty frame tree.
- Reattach the frame tree to the main frame when restoring it.
- open() the cached subframes in their respective loaders when restoring.
- Properly clear() and destroy() cached subframes.
- When committing to a new page load, and after caching the previous page, only clear the
  FrameView for the main frame.

Note that all of above will eventually be necessary as I continue to land this work in pieces,
but doesn't actually change any behavior right now because we still refuse to even *try* to
cache pages with frames.

No new tests. No change in behavior, and current tests continue to pass.

* history/CachedFrame.cpp:
(WebCore::CachedFrame::CachedFrame):
(WebCore::CachedFrame::restore):
(WebCore::CachedFrame::clear):
(WebCore::CachedFrame::destroy):
* history/CachedFrame.h:
(WebCore::CachedFrame::isMainFrame):

* loader/FrameLoader.cpp:
(WebCore::FrameLoader::clear):
(WebCore::FrameLoader::open):
* loader/FrameLoader.h:



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@47985 268f45cc-cd09-0410-ab3c-d52691b4dbfc