chromeos: Change SingleProcessMash shelf label to 'SPM'

I accidentally committed a debugging version where it was 'a'.

Bug: none
Change-Id: Ic544a6a78d48993dd0b17f05c17f4f616b59ea6a
Reviewed-on: https://chromium-review.googlesource.com/c/1289294
Commit-Queue: James Cook <jamescook@chromium.org>
Commit-Queue: Michael Wasserman <msw@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600844}

ash/system/flag_warning/flag_warning_tray.cc

@@ -83,7 +83,7 @@ void FlagWarningTray::UpdateButton() {
   }
 
   if (::features::IsSingleProcessMash()) {
-    button_->SetText(base::ASCIIToUTF16("a"));
+    button_->SetText(base::ASCIIToUTF16("SPM"));
     button_->SetTooltipText(
         base::ASCIIToUTF16("Running with feature SingleProcessMash"));
   } else if (::features::IsMultiProcessMash()) {