Makes instant throw out the first navigation when merging navigation

      stacks. This is because the first navigation is for the home page, and
generally we don't want that in the navigation stack.

I kept the remove first entry functionality in CopyStateFromAndPrune
as its easy to use wrong and I don't want to open up a general API.

BUG=74945
TEST=see bug
R=brettw@chromium.org

Review URL: http://codereview.chromium.org/6709039

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@78880 0039d316-1c4b-4281-b951-d872f2087c98