[Sync] Add support for identifying tabbed activites

components/sync/protocol/session_specifics.proto

Index: components/sync/protocol/session_specifics.proto
diff --git a/components/sync/protocol/session_specifics.proto b/components/sync/protocol/session_specifics.proto
index 8c9ebfb37d4850e59d20d6b90a2979f9e19d3dca..cedd4723dac6e83704b1b11a598be24ca7ba80cb 100644
--- a/components/sync/protocol/session_specifics.proto
+++ b/components/sync/protocol/session_specifics.proto
@@ -40,11 +40,12 @@ message SessionWindow {
   optional int32 window_id = 1;
   // Index of the selected tab in tabs; -1 if no tab is selected.
   optional int32 selected_tab_index = 2 [default = -1];
-  // Type of the browser. Currently we only store browsers of type
-  // TYPE_TABBED and TYPE_POPUP.
+  // Type of the window.
   enum BrowserType {

[Ted C, 2016/11/08 00:41:45]

I'll start with naive questioning here.

Can this be exposed to stuff outside of sync?  Could it then be exposed to java
(I don't know if we generate java code from our protos or not).

Ideally, I think we would expose GetSyncBrowserType in the sync delegate and we
could pass this down.

In general, I would like to avoid the instanceof check of the
TabbedModeTabPersistencePolicy because that really shouldn't belong in the
package that it is in and I want to avoid any new dependencies.

My "hope" would be that we could expose this to java, then the constructor of
TabModelSelectorImpl and TabModelImpl would take the sync type and pass it to
jni.

If we can't expose the constant, I think a boolean isTabbedTypeForSync or
something similarly gross and long would be my second option.

[Nicolas Zea, 2016/11/08 20:52:40]

Hmm. I think there's a question of whether the TabModel should be aware of Tab
Sync semantics, or whether Tab Sync should be aware of Tab semantics. I lean
towards the latter, as Tab Sync already depends on window/tab details, and the
tab model really doesn't (and likely won't in the future) need to know anything
about Sync proto details. As such, having some way of asking the tab model "what
kind of activity are you in" is what I was thinking.

That said, we do in fact generate java code from protos, at least for
instrumentation tests (org.chromium.components.sync.protocol). Just so I
understand, what would eventually implement the GetSyncBrowserType method, and
how would it determine what to return?

I'm fine with using something different than instanceof (e.g. plumbing something
in to the TabModelSelectorImpl at construction time) if that's preferable.

-    TYPE_TABBED = 1;
-    TYPE_POPUP = 2;
+    TYPE_TABBED = 1;      // A normal tabbed window.
+    TYPE_POPUP = 2;       // A popup window.
+    TYPE_CUSTOM_TAB = 3;  // A custom tab within an app. Not restored
+                          // automatically on sync startup.
   }
   optional BrowserType browser_type = 3 [default = TYPE_TABBED];
   // The tabs that compose a window (correspond to tab id's).