Ensure that child FrameTreeNodes are deleted after a navigation or crash.

BUG=245126
TEST=FrameTreeNodes deleted after a renderer crash.

Review URL: https://codereview.chromium.org/90413002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237601 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237601