about:net-internals#prerender:  Add information
about whether a prerender has finished loading or not.

Also wait until a prerender has finished loading in the
browser test before swapping in the prerender.  This is
needed before https://codereview.chromium.org/15041004/
can be landed (Which prevents a prerender from being
swapped in before commit, to fix a bug).

BUG=234809

Review URL: https://chromiumcodereview.appspot.com/16194008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@203420 0039d316-1c4b-4281-b951-d872f2087c98
3 files changed