Mount all partitions before running postinstall

For Virtual AB Compression, we need to mount snapshotted partitions
manually.

Test: treehugger
Change-Id: I15af8375e393ce2539d2075b467e8dbe94bec247
diff --git a/metrics_utils.cc b/metrics_utils.cc
index 2211a67..a947a7e 100644
--- a/metrics_utils.cc
+++ b/metrics_utils.cc
@@ -95,6 +95,7 @@
     case ErrorCode::kPostinstallRunnerError:
     case ErrorCode::kPostinstallBootedFromFirmwareB:
     case ErrorCode::kPostinstallFirmwareRONotUpdatable:
+    case ErrorCode::kPostInstallMountError:
       return metrics::AttemptResult::kPostInstallFailed;
 
     case ErrorCode::kUserCanceled:
@@ -189,6 +190,7 @@
     case ErrorCode::kOmahaResponseHandlerError:
     case ErrorCode::kFilesystemCopierError:
     case ErrorCode::kPostinstallRunnerError:
+    case ErrorCode::kPostInstallMountError:
     case ErrorCode::kPayloadMismatchedType:
     case ErrorCode::kInstallDeviceOpenError:
     case ErrorCode::kKernelDeviceOpenError: