Add keepalive support to drivefs API

In some situations mounting drivefs may take very long time. To
distinguish it from a total hang we send periodic keepalives from drivefs.

BUG=chromium:899746

Change-Id: Iee906651557a8f8eab62d58298f33c7c3e61724e
Reviewed-on: https://chromium-review.googlesource.com/c/1305253
Commit-Queue: Sergei Datsenko <dats@chromium.org>
Reviewed-by: Sam McNally <sammc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#603732}

chromeos/components/drivefs/drivefs_host.cc

@@ -281,6 +281,13 @@ class DriveFsHost::MountState
     token_fetch_attempted_ = true;
   }
 
+  void OnHeartbeat() override {
+    DCHECK_CALLED_ON_VALID_SEQUENCE(host_->sequence_checker_);
+    if (host_->timer_->IsRunning()) {
+      host_->timer_->Reset();
+    }
+  }
+
   void OnMounted() override {
     DCHECK_CALLED_ON_VALID_SEQUENCE(host_->sequence_checker_);
     drivefs_has_mounted_ = true;

chromeos/components/drivefs/mojom/drivefs.mojom

@@ -95,6 +95,9 @@ interface DriveFsDelegate {
 
   // Invoked whenever a Team Drive root changes or gets deleted.
   OnTeamDriveChanged(string team_drive_id, CreateOrDelete change_type);
+
+  // Invoked from time to time to indicate that drivefs is operational.
+  OnHeartbeat();
 };
 
 struct DriveFsConfiguration {