https://bugs.webkit.org/show_bug.cgi?id=119781

Reviewed by Alexey Proskuryakov.

Source/WebCore:

We should pair restoring a cached frame and adding it to FrameTree so that
actual added frame count in FrameTree keeps same with restored frame count.
Otherwise assert failure occurs in Page::CheckSubframeCountConsistency.
We have added cached frames to FrameTree ahead and then restored them by tree travelsal.
But this way doesn't guarantee subframecount of Page and the number of child of FrameTree are equal.
In addition, we still make sure that adding a cached frame to FrameTree should happen
before opening it. Because FrameTree might be used while restoring a cached page.

Test: fast/history/go-back-to-iframe-with-plugin.html

* history/CachedFrame.cpp:
(WebCore::CachedFrameBase::restore):
(WebCore::CachedFrame::open):

LayoutTests:

To verify this fix, we need to use the CachedPage in WK2 debug mode.
And the test page should have at least one iframe with plugin.

* fast/history/go-back-to-iframe-with-plugin-expected.txt: Added.
* fast/history/go-back-to-iframe-with-plugin.html: Added.
* fast/history/resources/subframe-with-plugin.html: Added.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@155361 268f45cc-cd09-0410-ab3c-d52691b4dbfc