IndexedDB: Add missing UMA enum mapping for ForceCloseReason.

In 48bedbd2 a new reason was added to force an Indexed DB closed:
for migration. This change failed to add a value in enums.xml for UMA.

Not strictly related to 643544, but was noticed when reviewing UMA
values post change.

Bug: 643544
Change-Id: I793df6967a80046775babb1cf0af242b2c148374
Reviewed-on: https://chromium-review.googlesource.com/792031Reviewed-by: Joshua Bell <jsbell@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Commit-Queue: Chris Mumford <cmumford@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519756}

content/browser/indexed_db/indexed_db_context_impl.h

@@ -47,6 +47,8 @@ class CONTENT_EXPORT IndexedDBContextImpl : public IndexedDBContext {
     FORCE_CLOSE_BACKING_STORE_FAILURE,
     FORCE_CLOSE_INTERNALS_PAGE,
     FORCE_CLOSE_COPY_ORIGIN,
+    // Append new values here and update IDBContextForcedCloseReason in
+    // enums.xml.
     FORCE_CLOSE_REASON_MAX
   };
 

tools/metrics/histograms/enums.xml

@@ -21325,6 +21325,9 @@ Called by update_gpu_driver_bug_workaround_entries.py.-->
   <int value="2" label="InternalsPage">
     A forced close was requested from the indexeddb-internals page.
   </int>
+  <int value="3" label="CopyOrigin">
+    The database is force closed so that it can be copied.
+  </int>
 </enum>
 
 <enum name="IDBException">