[IndexedDB]: Add use counter to IDB databases.

Adds a UMA use counter to IDBFactory.databases().

Bug: 909000
Change-Id: I3a45c7cccec208fa8d9227bcfb2cc24a4a688324
Reviewed-on: https://chromium-review.googlesource.com/c/1352484Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Commit-Queue: Andreas Butler <andreasbutler@google.com>
Cr-Commit-Position: refs/heads/master@{#612442}

third_party/blink/public/platform/web_feature.mojom

@@ -2097,6 +2097,7 @@ enum WebFeature {
   kCSSSelectorNotWithValidList = 2645,
   kCSSSelectorNotWithInvalidList = 2646,
   kCSSSelectorNotWithPartiallyValidList = 2647,
+  kV8IDBFactory_Databases_Method = 2648,
 
   // Add new features immediately above this line. Don't change assigned
   // numbers of any item, and don't reuse removed slots.

third_party/blink/renderer/modules/indexeddb/idb_factory.idl

@@ -40,6 +40,7 @@
     [
       CallWith=ScriptState,
       ImplementedAs=GetDatabaseInfo,
+      Measure,
       RaisesException
     ] Promise<sequence<IDBDatabaseInfo>> databases();
 };

tools/metrics/histograms/enums.xml

@@ -20895,6 +20895,7 @@ Called by update_net_error_codes.py.-->
   <int value="2645" label="CSSSelectorNotWithValidList"/>
   <int value="2646" label="CSSSelectorNotWithInvalidList"/>
   <int value="2647" label="CSSSelectorNotWithPartiallyValidList"/>
+  <int value="2648" label="V8IDBFactory_Databases_Method"/>
 </enum>
 
 <enum name="FeaturePolicyFeature">