Log mixed script shield events.

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 01e6809e03ca3e89f81aeb43d388576ee5f0ddf2..5e22b75a3a59eecae5428897d1f25f7a7b3ee3ba 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -3064,6 +3064,15 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="ContentSettings.MixedScript"
+    enum="ContentSettingMixedScriptAction">
+  <owner>lgarron@chromium.org</owner>
+  <summary>
+    Tracks whether the mixed content shield was shown, and how the user
+    interacted with it.
+  </summary>
+</histogram>
+
 <histogram name="ContentSettings.PermissionActions" enum="PermissionAction">
   <owner>miguelg@chromium.org</owner>
   <summary>
@@ -39807,6 +39816,13 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   <int value="4" label="SESSION_ONLY"/>
 </enum>
 
+<enum name="ContentSettingMixedScriptAction" type="int">
+  <int value="0" label="Displayed shield"/>
+  <int value="1" label="Displayed bubble"/>
+  <int value="2" label="Clicked 'Load unsafe scripts'"/>
+  <int value="3" label="Clicked 'Learn more'"/>
+</enum>
+
 <enum name="CookieDeletionCause" type="int">
   <summary>Reason why a cookie was removed from the cookie store</summary>
   <int value="0" label="explicit">