Add UMA to track the extensions (and friends) which have been allowed in

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index 5c8ba98eaa4111d7a69aacb742a3bd4df9a57499..0b8e1705490be7ebe191515351cb18a325bdd2a4 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -5978,6 +5978,27 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="Extensions.FileAccessAllowed">
+  <owner>kalman@chromium.org</owner>
+  <summary>
+    The number of installed extensions (and friends) that could have been given
+    access to the file:// scheme, and were. This excludes entities such as
+    platform apps (which don't have host permissions) and hosted apps (which
+    aren't shown on chrome://extensions). See also
+    Extensions.FileAccessNotAllowed.
+  </summary>
+</histogram>
+
+<histogram name="Extensions.FileAccessNotAllowed">
+  <owner>kalman@chromium.org</owner>
+  <summary>
+    The number of installed extensions (and friends) that could have been given
+    access to the file:// scheme, but weren't. This excludes entities such as
+    platform apps (which don't have host permissions) and hosted apps (which
+    aren't shown on chrome://extensions). See also Extensions.FileAccessAllowed.
+  </summary>
+</histogram>
+
 <histogram name="Extensions.FromWebstoreInconsistency"
     enum="ExtensionFromWebstoreInconcistencyEnum">
   <owner>Please list the metric's owners. Add more owner tags as needed.</owner>
@@ -6001,6 +6022,26 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="Extensions.IncognitoAllowed">
+  <owner>kalman@chromium.org</owner>
+  <summary>
+    The number of installed extensions (and friends) that could have been
+    enabled in incognito, and were. This excludes entities such as platform apps
+    (which cannot be enabled) and hosted apps (which aren't shown on
+    chrome://extensions). See also Extensions.IncognitoNotAllowed.
+  </summary>
+</histogram>
+
+<histogram name="Extensions.IncognitoNotAllowed">
+  <owner>kalman@chromium.org</owner>
+  <summary>
+    The number of installed extensions (and friends) that could have been
+    enabled in incognito, but weren't. This excludes entities such as platform
+    apps (which cannot be enabled) and hosted apps (which aren't shown on
+    chrome://extensions). See also Extensions.IncognitoAllowed.
+  </summary>
+</histogram>
+
 <histogram name="Extensions.InjectCssTime" units="milliseconds">
   <owner>Please list the metric's owners. Add more owner tags as needed.</owner>
   <summary>