Add some <base> metrics to guide experimentation.

third_party/WebKit/Source/core/frame/UseCounter.h

 /*
  * Copyright (C) 2012 Google, Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 1407 matching lines @@
     PromptEngagementMax = 1748,
     TopNavInSandbox = 1749,
     TopNavInSandboxWithoutGesture = 1750,
     TopNavInSandboxWithPerm = 1751,
     TopNavInSandboxWithPermButNoGesture = 1752,
     ReferrerPolicyHeader = 1753,
     HTMLAnchorElementReferrerPolicyAttribute = 1754,
     HTMLIFrameElementReferrerPolicyAttribute = 1755,
     HTMLImageElementReferrerPolicyAttribute = 1756,
     HTMLLinkElementReferrerPolicyAttribute = 1757,
+    BaseElement = 1758,
+    BaseWithCrossOriginHref = 1759,
+    BaseWithDataHref = 1760,
+    BaseWithNewlinesInTarget = 1761,
+    BaseWithOpenBracketInTarget = 1762,
+    BaseWouldBeBlockedByDefaultSrc = 1763,
 
     // Add new features immediately above this line. Don't change assigned
     // numbers of any item, and don't reuse removed slots.
     // Also, run update_use_counter_feature_enum.py in
     // chromium/src/tools/metrics/histograms/ to update the UMA mapping.
     NumberOfFeatures,  // This enum value must be last.
   };
 
   // "count" sets the bit for this feature to 1. Repeated calls are ignored.
   static void count(const Frame*, Feature);
@@ skipping 68 matching lines @@
     // Tracks what features/properties need to be reported to the legacy
     // histograms.
     BitVector m_featureBits;
     BitVector m_CSSBits;
   } m_legacyCounter;
 };
 
 }  // namespace blink
 
 #endif  // UseCounter_h