predictors: Make speculative_prefetch_predictor work with PlzNavigate

chrome/browser/predictors/resource_prefetch_common.h

Index: chrome/browser/predictors/resource_prefetch_common.h
diff --git a/chrome/browser/predictors/resource_prefetch_common.h b/chrome/browser/predictors/resource_prefetch_common.h
index afbd9240545ce0846f09f6818db313f583d4ee85..151bdc80529b02641c97248613e4778901c8f4e6 100644
--- a/chrome/browser/predictors/resource_prefetch_common.h
+++ b/chrome/browser/predictors/resource_prefetch_common.h
@@ -8,6 +8,7 @@
 #include <stddef.h>
 
 #include "base/time/time.h"
+#include "components/sessions/core/session_id.h"
 #include "url/gurl.h"
 
 class Profile;
@@ -43,17 +44,15 @@ struct NavigationID {
                const GURL& main_frame_url,
                const base::TimeTicks& creation_time);
   NavigationID(const NavigationID& other);
+
   bool operator<(const NavigationID& rhs) const;
   bool operator==(const NavigationID& rhs) const;
 
-  bool IsSameRenderer(const NavigationID& other) const;
-
   // Returns true iff the render_process_id_, render_frame_id_ and
   // frame_url_ has been set correctly.

[clamy, 2016/12/22 15:52:41]

This comment needs to be updated since render_process_id_ and render_frame_id_
no longer exist.

[ahemery, 2016/12/22 17:12:26]

Done.

   bool is_valid() const;
 
-  int render_process_id;
-  int render_frame_id;
+  SessionID::id_type tab_id;

[clamy, 2016/12/22 15:52:41]

How do you deal with the two concurrent navigation in the main frame case? They
would have been distinguished by the (render_process_id, render_frame_id)
before. IIUC the predictor code correctly, you are only listening for events on
the IO thread by observing the ResourceDispatcherHostDelegate. With PlzNavigate
enabled, it is not possible to know which of two concurrent navigations will
commit only by listening to IO thread events. You need to listen to
WebContentsObserver events on the UI thread.

While we're talking about special cases in navigation, how do you handle
client-side redirects? Are they coalesced with the previous navigation or
treated as a different navigation? IMO there's a rationale for saying they
should be coalesced with the previous navigation.

[Benoit L, 2016/12/22 16:04:10]

Per offline discussion:
- The NavigationID matches not only on the IDs, so this should be fine.
- Client-side redirects are not handled yet, but that's a nice addition for a
future patch, thanks for letting us know.

   GURL main_frame_url;
 
   // NOTE: Even though we store the creation time here, it is not used during