Update failure reason UNKNOWN field according to guidelines

During the sync of the protos on client side and server side, it
was requested to change UNKNOWN field to FAILURE_REASON_UNKNOWN to
avoid confusion in the future and to be in sync with the
guidelines.

Bug: 1106343
Change-Id: I40c6a3e4a0299dfa7a8360dcc44bb09292ee57b9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2336079Reviewed-by: Sergey Poromov <poromov@chromium.org>
Commit-Queue: Swapnil Gupta <swapnilgupta@google.com>
Cr-Commit-Position: refs/heads/master@{#794541}

chrome/browser/chromeos/policy/extension_install_event_log_collector.cc

@@ -51,7 +51,7 @@ em::ExtensionInstallReportLogEvent_FailureReason ConvertFailureReasonToProto(
     extensions::InstallStageTracker::FailureReason failure_reason) {
   switch (failure_reason) {
     case extensions::InstallStageTracker::FailureReason::UNKNOWN:
-      return em::ExtensionInstallReportLogEvent::UNKNOWN;
+      return em::ExtensionInstallReportLogEvent::FAILURE_REASON_UNKNOWN;
     case extensions::InstallStageTracker::FailureReason::INVALID_ID:
       return em::ExtensionInstallReportLogEvent::INVALID_ID;
     case extensions::InstallStageTracker::FailureReason::

components/policy/proto/device_management_backend.proto

@@ -2832,7 +2832,7 @@ message ExtensionInstallReportLogEvent {
   // details. InstallStageTracker::FailureReason is the main enum and this is
   // a copy used for reporting purposes.
   enum FailureReason {
-    UNKNOWN = 0;
+    FAILURE_REASON_UNKNOWN = 0;
 
     INVALID_ID = 1;