Revert "Disable state-preserving TPM firmware update."

This reverts commit bcbd1740.

Reason for revert: The bug that prompted disabling of the state-preserving firmware update feature has been fixed per https://chromium-review.googlesource.com/1123830 so we can re-enable the feature.

Original change's description:
> Disable state-preserving TPM firmware update.
> 
> The test team found an issue where cryptohomed fails to deal with the
> state the TPM is in for the case when the update installation gets
> interrupted and recovery is invoked to retry and complete the update.
> Disable device-state preserving updates for now until the issue is
> resolved.
> 
> BUG=chromium:854576
> TEST=Triggering TPM firmware update via chrome://chrome will not invoke the device state preserving update flow.
> 
> Change-Id: I4ac4d325bfbf953cfa14a501e871a3f9f59c699c
> Reviewed-on: https://chromium-review.googlesource.com/1111998
> Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
> Commit-Queue: Mattias Nissler <mnissler@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#570020}

TBR=stevenjb@chromium.org,mnissler@chromium.org

Bug: chromium:854576
Change-Id: Iab59dc64130792b92071d45b82a77ddbd4553aa4
Reviewed-on: https://chromium-review.googlesource.com/1127539Reviewed-by: Mattias Nissler <mnissler@chromium.org>
Commit-Queue: Mattias Nissler <mnissler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578570}

chrome/browser/ui/webui/settings/browser_lifetime_handler.cc

@@ -30,9 +30,7 @@ void TriggerTPMFirmwareUpdate(
   using chromeos::tpm_firmware_update::Mode;
 
   // Decide which update mode to use.
-  // TODO(crbug.com/854576): Re-add Mode::kPreserveDeviceState after fixing
-  // interrupted update flow issue described in the bug.
-  for (Mode mode : {Mode::kPowerwash}) {
+  for (Mode mode : {Mode::kPreserveDeviceState, Mode::kPowerwash}) {
     if (available_modes.count(mode) == 0) {
       continue;
     }