Clear mount_start_ when deferring DriveFS startup due to being offline.

For the first DriveFS, mount, mount_start_ is retained across mount
attempts to record the total time taken to launch. When the DriveFS
mount fails while offline, retries are delayed until online again. If a
retry for the first launch is delayed in this fashion, the original
start time is retained, causing spurious long first mount time metrics
to be reported. Avoid this by clearing mount_start_ when delaying
retries due to being offline. Clear it when disabling drive too.

Bug: 892524
Change-Id: I47db55ca5b17965267ef813e2e70b7f7d90b662a
Reviewed-on: https://chromium-review.googlesource.com/c/1322345Reviewed-by: Sergei Datsenko <dats@chromium.org>
Commit-Queue: Sam McNally <sammc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606024}

chrome/browser/chromeos/drive/drive_integration_service.cc

@@ -451,6 +451,7 @@ class DriveIntegrationService::PreferenceWatcher
     if (integration_service_->drivefs_holder_ &&
         integration_service_->remount_when_online_ && !is_offline) {
       integration_service_->remount_when_online_ = false;
+      integration_service_->mount_start_ = {};
       integration_service_->AddDriveMountPoint();
       return;
     }
@@ -761,6 +762,7 @@ void DriveIntegrationService::SetEnabled(bool enabled) {
     drivefs_total_failures_count_ = 0;
     drivefs_consecutive_failures_count_ = 0;
     mount_failed_ = false;
+    mount_start_ = {};
   }
 }