Make it possible for the embedder to run unload handlers in subframes.

Make it possible for the embedder to run unload handlers in subframes.

This is needed to run unload handlers in subframes before the IPC filter
is installed for cross-process swaps.

BUG=

[dcheng, 2015-04-10 18:22:37]

dcheng@chromium.org changed reviewers:
+ alexmos@chromium.org, creis@chromium.org

[dcheng, 2015-04-10 18:22:38]

This is one way to fix the problem. There are others.

[dcheng, 2015-04-10 18:24:10]

Also, this patch needs to be rebased to ToT. The function called have moved
around since then.

If we feel this patch is the right direction, I'll go ahead and update it.

[alexmos, 2015-04-10 23:41:43]

On 2015/04/10 18:24:10, dcheng wrote:
> Also, this patch needs to be rebased to ToT. The function called have moved
> around since then.
> 
> If we feel this patch is the right direction, I'll go ahead and update it.

I'm all for something like this.  To me it makes much more sense to run subframe
unload handlers together with the main frame's unload rather than when
navigating to swappedout:// after swapped_out_ becomes true.  And we'll need
something like this anyway to fire the subframe unload handlers when we get rid
of swappedout://, right?