Add some <base> metrics to guide experimentation.

third_party/WebKit/Source/core/frame/csp/CSPDirectiveList.cpp

Index: third_party/WebKit/Source/core/frame/csp/CSPDirectiveList.cpp
diff --git a/third_party/WebKit/Source/core/frame/csp/CSPDirectiveList.cpp b/third_party/WebKit/Source/core/frame/csp/CSPDirectiveList.cpp
index 6b7bbc7b4f277e446097753857aaaea1d56499ee..d3a63a2f37e0fb9940067aa4ac934e29f7825013 100644
--- a/third_party/WebKit/Source/core/frame/csp/CSPDirectiveList.cpp
+++ b/third_party/WebKit/Source/core/frame/csp/CSPDirectiveList.cpp
@@ -760,12 +760,20 @@ bool CSPDirectiveList::allowBaseURI(
     const KURL& url,
     ResourceRequest::RedirectStatus redirectStatus,
     ContentSecurityPolicy::ReportingStatus reportingStatus) const {
-  return reportingStatus == ContentSecurityPolicy::SendReport
-             ? checkSourceAndReportViolation(
-                   m_baseURI.get(), url,
-                   ContentSecurityPolicy::DirectiveType::BaseURI,
-                   redirectStatus)
-             : checkSource(m_baseURI.get(), url, redirectStatus);
+  bool result =
+      reportingStatus == ContentSecurityPolicy::SendReport
+          ? checkSourceAndReportViolation(
+                m_baseURI.get(), url,
+                ContentSecurityPolicy::DirectiveType::BaseURI, redirectStatus)
+          : checkSource(m_baseURI.get(), url, redirectStatus);
+
+  if (result &&
+      !checkSource(operativeDirective(m_baseURI.get()), url, redirectStatus)) {
+    UseCounter::count(m_policy->document(),
+                      UseCounter::BaseWouldBeBlockedByDefaultSrc);
+  }
+
+  return result;
 }
 
 bool CSPDirectiveList::allowWorkerFromSource(