media: Add UMA to record whether encrypted media is enabled

The UMA is recorded when requestMediaKeySystemAccess is called on
non-ClearKey key system but will reported at most once per renderer
process.

BUG=711348
TEST=Manually tested

Review-Url: https://codereview.chromium.org/2824643002
Cr-Commit-Position: refs/heads/master@{#465159}

third_party/WebKit/Source/modules/encryptedmedia/NavigatorRequestMediaKeySystemAccess.cpp

@@ -278,9 +278,20 @@ ScriptPromise NavigatorRequestMediaKeySystemAccess::requestMediaKeySystemAccess(
   //
   // Do not check settings for Clear Key.
   if (key_system != "org.w3.clearkey") {
-    // For other key systems, check settings.
-    if (!document->GetSettings() ||
-        !document->GetSettings()->GetEncryptedMediaEnabled()) {
+    // For other key systems, check settings and report UMA.
+    bool encypted_media_enabled =
+        document->GetSettings() &&
+        document->GetSettings()->GetEncryptedMediaEnabled();
+
+    static bool has_reported_uma = false;
+    if (!has_reported_uma) {
+      has_reported_uma = true;
+      DEFINE_STATIC_LOCAL(BooleanHistogram, histogram,
+                          ("Media.EME.EncryptedMediaEnabled"));
+      histogram.Count(encypted_media_enabled);
+    }
+
+    if (!encypted_media_enabled) {
       return ScriptPromise::RejectWithDOMException(
           script_state,
           DOMException::Create(kNotSupportedError, "Unsupported keySystem"));

tools/metrics/histograms/histograms.xml

@@ -26939,6 +26939,16 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Media.EME.EncryptedMediaEnabled" enum="BooleanEnabled">
+  <owner>xhwang@chromium.org</owner>
+  <summary>
+    Whether encrypted media is enabled when requestMediaKeySystemAccess() is
+    called. User can enable and disable encrypted media in content settings.
+    Recorded when requestMediaKeySystemAccess() is called but will be reported
+    at most once per renderer process.
+  </summary>
+</histogram>
+
 <histogram name="Media.EME.IsIncognito" enum="BooleanIncognito">
   <owner>xhwang@chromium.org</owner>
   <summary>