Reviewed by Darin Adler.

        Minimize memory due to layer backing stores for pages in the Page Cache
        https://bugs.webkit.org/show_bug.cgi?id=62675

        Test the layer tree of a page with composited content,
        including content in iframes, looks like we expect.
        Enabling clearing backing stores in the PageCache should
        produce the same expected results.

        * compositing/iframes/page-cache-layer-tree-expected.txt: Added.
        * compositing/iframes/page-cache-layer-tree.html: Added.
        * compositing/iframes/resources/page-cache-helper.html: Added.
        * compositing/iframes/resources/page-cache-iframe.html: Added.

2011-06-20  Joseph Pecoraro  <joepeck@webkit.org>

        Reviewed by Simon Fraser.

        Minimize memory due to layer backing stores for pages in the Page Cache
        https://bugs.webkit.org/show_bug.cgi?id=62675

        When a page enters the page cache there is now an option on
        the PageCache singleton to clear the backing stores and layers
        of the cached page. This can be useful to minimize the amount
        of memory the stored page consumes.

        This only affects memory usage and is disabled by default, so no test.

        * history/CachedFrame.h:
        * history/CachedFrame.cpp:
        (WebCore::CachedFrameBase::CachedFrameBase):
        Save whether or not the page had composited content or not
        in a new instance variable, m_isComposited.

        (WebCore::CachedFrameBase::restore):
        When restoring, rebuild the compositing tree if it may have
        been destroyed.

        (WebCore::CachedFrame::CachedFrame):
        When saving, clear the backing stores if the page is in
        compositing mode, and the PageCache setting is enabled.

        * history/PageCache.cpp:
        (WebCore::PageCache::PageCache):
        * history/PageCache.h:
        (WebCore::PageCache::shouldClearBackingStores):
        (WebCore::PageCache::setShouldClearBackingStores):
        A setting to opt-in to the new behavior of clearing
        the backing stores.

        * page/FrameView.h:
        * page/FrameView.cpp:
        (WebCore::FrameView::clearBackingStores):
        (WebCore::FrameView::restoreBackingStores):
        To clear all backing stores we disable compositing
        for the frame, to detach the root platform layer,
        and recursively clear backing stores from the root
        layer of the FrameView.

        * rendering/RenderLayer.h:
        * rendering/RenderLayer.cpp:
        (WebCore::RenderLayer::clearBackingIncludingDescendants):
        Recursively call clearBacking.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@89316 268f45cc-cd09-0410-ab3c-d52691b4dbfc