histograms: Add "Crash.Sender.FailedCrashRemoval"

Add a new UMA metric to CrosEventEnum to assist in debugging an issue
where crash_sender sends the same crash repeatedly.

BUG=chromium:1060019
TEST=CQ

Change-Id: Ia57bd32f954b22677ad8e23c4f221a656c883510
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2311414
Commit-Queue: Miriam Zimmerman <mutexlox@chromium.org>
Reviewed-by: Ian Barkley-Yeung <iby@chromium.org>
Cr-Commit-Position: refs/heads/master@{#790661}

tools/metrics/histograms/enums.xml

@@ -13520,6 +13520,7 @@ to ensure that the crash string is shown properly on the user-facing crash UI.
   <int value="27" label="Crash.Collector.CollectionCount"/>
   <int value="28" label="Cryptohome.DoubleMountRequest"/>
   <int value="29" label="SessionManager.SafeModeEnabled"/>
+  <int value="30" label="Crash.Sender.FailedCrashRemoval"/>
 </enum>
 
 <enum name="CrosFirstRunTutorialCompletionType">