Replace PruneAllButActive with PruneAllButVisible.

chrome/test/data/webui/net_internals/prerender_view.js

Index: chrome/test/data/webui/net_internals/prerender_view.js
diff --git a/chrome/test/data/webui/net_internals/prerender_view.js b/chrome/test/data/webui/net_internals/prerender_view.js
index 6d4f57dd625debf597b0aaecdd24acb364d736fe..947f07aa7d7425ffc3db6c208b414f363b31f6ad 100644
--- a/chrome/test/data/webui/net_internals/prerender_view.js
+++ b/chrome/test/data/webui/net_internals/prerender_view.js
@@ -100,6 +100,7 @@ PrerenderTask.prototype = {
     } else if (this.state_ == STATE.NEED_NAVIGATE) {
       this.navigate_(prerenderInfo);

[cbentzel, 2013/05/28 18:49:11]

This is probably the location where we need to hold off navigating until the
prerendered page commits - rather than the checkDone_ as your TODO mentions
below. Otherwise, for "should be successful" prerenders we run the risk of
navigating too early and not swapping in the prerender.

Matt Menke is probably the best person to ask for how to pipe that information
in - he wrote all of this test infrastructure for net-internals.  

[mmenke, 2013/05/28 19:06:36]

It seems like the simplest thing to do is to wait until we've seen the
URLRequest for the prerender complete.  Since we're conveniently running in
net-internals, we can just watch for the corresponding NetLog events.  To do
this, you'd need to add |this| as an observer of the event stream, and hold off
on navigating until we see the start and end REQUEST_ALIVE events for the URL
request for this.url_.

Since the REQUEST_ALIVE event doesn't contain the URL, you'll have to check the
URL_REQUEST_START_JOB event to get the URL.

[mmenke, 2013/05/28 19:20:04]

Just realized this may not be waiting long enough, and even it if is, there's no
guarantee that won't regress.

One simple option is to include whether or not a page has finished loading in
the prerenderInfo data structure, and wait for that to switch to true.  We can
also add it to the table, just for kicks.

May be simplest to do this in another CL, and land it first. 

[Charlie Reis, 2013/05/28 23:22:00]

I follow you conceptually, but I'm not familiar with any of this code.  Where
does the prerenderInfo object come from, who would set the commit value on it,
and how would we wait for that to be true?

Doing this in a separate CL makes sense to me.  Matt, I'm happy to send one your
way if you can explain what needs to be done, unless you think it'll be quicker
to draft it yourself.  :)

[mmenke, 2013/05/28 23:38:20]

Yea, I think it's best I just do it myself.  Think it'll be a pretty quick CL,
and you don't really gain anything by learning the code well enough to do it.

     } else if (this.state_ == STATE.HISTORY_WAIT) {
+      // TODO(creis): How can we wait for commit before checkDone_?
       this.checkDone_(prerenderInfo);
     } else {
       assertNotReached();