histograms: Add Crash.Sender.AttemptedCrashRemoval

This metric will be used as a denominator for the recently-added
Crash.Sender.FailedCrashRemoval metric.

BUG=chromium:1060019
TEST=CQ

Change-Id: I7f2781cafcebe2b70e668d98ffde42403d432772
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2317297Reviewed-by: Ian Barkley-Yeung <iby@chromium.org>
Commit-Queue: Miriam Zimmerman <mutexlox@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791945}

tools/metrics/histograms/enums.xml

@@ -13537,6 +13537,7 @@ to ensure that the crash string is shown properly on the user-facing crash UI.
   <int value="28" label="Cryptohome.DoubleMountRequest"/>
   <int value="29" label="SessionManager.SafeModeEnabled"/>
   <int value="30" label="Crash.Sender.FailedCrashRemoval"/>
+  <int value="31" label="Crash.Sender.AttemptedCrashRemoval"/>
 </enum>
 
 <enum name="CrosFirstRunTutorialCompletionType">