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/1111998Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Mattias Nissler <mnissler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570020}

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

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