Fix transferring processes during a redirect.

Fix transferring processes during a redirect.

This change adds a WillCompleteTransfer method on ResourceLoader.  The
ResourceLoader uses this signal to reset the ResourceHandler, which causes the
destruction of the old AsyncResourceHandler.  That in turn results in clearing
the "async_handler" member of the ResourceRequestInfoImpl.

What was happening before this change is that the destruction of the old
AsyncResourceHandler was happening too late.  When it cleared the
"async_handler" member, it was clearing it on the newly constructed
ResourceRequestInfoImpl object.  As a result, when the OnFollowRedirect
IPC finally arrives, it finds that the "async_handler" member is null.

R=jam@chromium.org
BUG=136630
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=146416

[darin (slow to review), 2012-07-11 23:01:53]

I'm not quite sure how to write an automated test for this yet.  Apparently, the
only tests for this stuff is through the extension system, and for some reason
those tests didn't catch the regression.  I'd prefer a more localized test in
either resource_dispatcher_host_unittest.cc or
resource_dispatcher_host_browsertest.cc.

[darin (slow to review), 2012-07-12 07:04:10]

Now with a couple unit tests.  (Actually, these are really integration tests of
the ResourceDispatcherHost and associated ResourceHandlers.)

Two tests:
1-  Tests basic transfer of navigation upon redirect.
2-  Tests what the bug was about, namely that a redirect after transferring the
navigation is correctly followed (i.e., two redirects where the first redirect
causes a navigation to be transferred to a different process).

[darin (slow to review), 2012-07-12 07:06:11]

+mpcomplete,creis: FYI

[jam, 2012-07-12 18:54:37]

lgtm

[commit-bot: I haz the power, 2012-07-12 18:59:09]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/darin@chromium.org/10700170/1005