[wasm code cache] Label the new WriteFailed enum value.

- Gives a name to the CodeCacheStatus::WriteFailed enum value.

Bug: chromium:719172
Change-Id: I1291b8926877e6a782ebeb92b02331c49bd5ed96
Reviewed-on: https://chromium-review.googlesource.com/c/1480724Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634480}

tools/metrics/histograms/enums.xml

@@ -49225,6 +49225,9 @@ Called by update_net_trust_anchors.py.-->
     Entries that are read before a write to that entry has finished when read
     and write request interleave.
   </int>
+  <int value="7" label="WriteFailure">
+    Entries that could not be written, usually because they're too big.
+  </int>
 </enum>
 
 <enum name="SiteIsolatedCodeCacheWASMBehaviour">
@@ -49243,6 +49246,9 @@ Called by update_net_trust_anchors.py.-->
     Entries that are read before a write to that entry has finished when read
     and write request interleave.
   </int>
+  <int value="7" label="WriteFailure">
+    Entries that could not be written, usually because they're too big.
+  </int>
 </enum>
 
 <enum name="SiteIsolationMimeType">