lacros: Add an ash_chrome_service_version field to LacrosInitParams.

This will allow lacros to synchronously determine ash's version. This
allows Lacros to avoid using unsupported APIs.

Bug: 1130810
Change-Id: I57fb355861b70b082499e123606f97366e0c3670
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2423203Reviewed-by: Greg Kerr <kerrnel@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: Erik Chen <erikchen@chromium.org>
Auto-Submit: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809533}

chrome/browser/chromeos/crosapi/browser_util.cc

@@ -120,7 +120,10 @@ SendMojoInvitationToLacrosChrome(
           invitation.AttachMessagePipe(0 /* token */), /*version=*/0));
   lacros_chrome_service.set_disconnect_handler(
       std::move(mojo_disconnected_callback));
-  lacros_chrome_service->Init(crosapi::mojom::LacrosInitParams::New());
+  auto params = crosapi::mojom::LacrosInitParams::New();
+  params->ash_chrome_service_version =
+      crosapi::mojom::AshChromeService::Version_;
+  lacros_chrome_service->Init(std::move(params));
   lacros_chrome_service->RequestAshChromeServiceReceiver(
       std::move(ash_chrome_service_callback));
   mojo::OutgoingInvitation::Send(std::move(invitation),

chromeos/crosapi/mojom/crosapi.mojom

@@ -39,10 +39,28 @@ interface AshChromeService {
 };
 
 // LacrosInitParams is a set of parameters for initialization of lacros-chrome,
-// which is passed from ash-chrome.
+// which is passed from ash-chrome to lacros-chrome. Since ash-chrome and
+// lacros-chrome may have different versions, lacros-chrome must handle this
+// struct carefully.
+//
+// If ash-chrome is older than lacros-chrome, then some fields may not be
+// present in the serialized IPC message. This manifests as the newer (missing)
+// fields taking on default values in lacros-chrome. This means
+// that the default value for each field must always be interpreted to mean:
+// this field was not sent because ash-chrome was too old. If the default value
+// needs to have a different meaning, then we must also add a bool field
+// describing the validity of this field. Mojo currently does not support
+// optional primitives.
+//
+// If ash-chrome is newer than lacros-chrome, then some fields may not be
+// processed by lacros-chrome.
 [Stable]
 struct LacrosInitParams {
-  // This is placeholder, so now it is empty.
+  // This is ash-chrome's version of the AshChromeService interface. This is
+  // used by lacros-chrome to determine which interface methods are safe to
+  // call.
+  [MinVersion=1]
+  uint32 ash_chrome_service_version@0;
 };
 
 // LacrosChromeService defines the APIs that live in lacros-chrome and