Instrument MediaRecorder.{mimeType, videoBitsPerSecond,

audioBitsPerSecond} for identifiability study.

These web IDL annotations will automatically add instrumentation (using
blink::Dactyloscoper) in the bindings code to record how much entropy
these APIs leak about users' cross-site identity.

Bug: 973801
Change-Id: I6c61c13bb5c98f65415678f000e5137d016ba55c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2354153
Commit-Queue: Chris Fredrickson <cfredric@google.com>
Reviewed-by: Markus Handell <handellm@google.com>
Cr-Commit-Position: refs/heads/master@{#798266}

third_party/blink/public/mojom/web_feature/web_feature.mojom

@@ -2726,6 +2726,9 @@ enum WebFeature {
   kDigitalGoodsGetDigitalGoodsService = 3396,
   kDigitalGoodsGetDetails = 3397,
   kDigitalGoodsAcknowledge = 3398,
+  kMediaRecorder_MimeType = 3399,
+  kMediaRecorder_VideoBitsPerSecond = 3400,
+  kMediaRecorder_AudioBitsPerSecond = 3401,
 
   // Add new features immediately above this line. Don't change assigned
   // numbers of any item, and don't reuse removed slots.

third_party/blink/renderer/modules/mediarecorder/media_recorder.idl

@@ -12,7 +12,7 @@ enum RecordingState { "inactive", "recording", "paused" };
 ] interface MediaRecorder : EventTarget {
     [CallWith=ExecutionContext, RaisesException] constructor(MediaStream stream, optional MediaRecorderOptions options = {});
     readonly attribute MediaStream stream;
-    readonly attribute DOMString mimeType;
+    [HighEntropy=Direct, MeasureAs=MediaRecorder_MimeType] readonly attribute DOMString mimeType;
     readonly attribute RecordingState state;
 
     attribute EventHandler onstart;
@@ -21,8 +21,8 @@ enum RecordingState { "inactive", "recording", "paused" };
     attribute EventHandler onpause;
     attribute EventHandler onresume;
     attribute EventHandler onerror;
-    readonly attribute unsigned long videoBitsPerSecond;
-    readonly attribute unsigned long audioBitsPerSecond;
+    [HighEntropy=Direct, MeasureAs=MediaRecorder_VideoBitsPerSecond] readonly attribute unsigned long videoBitsPerSecond;
+    [HighEntropy=Direct, MeasureAs=MediaRecorder_AudioBitsPerSecond] readonly attribute unsigned long audioBitsPerSecond;
 
     [RaisesException, Measure] void start(optional long timeslice);
     [RaisesException] void stop();

tools/metrics/histograms/enums.xml

@@ -28739,6 +28739,9 @@ Called by update_use_counter_feature_enum.py.-->
   <int value="3396" label="DigitalGoodsGetDigitalGoodsService"/>
   <int value="3397" label="DigitalGoodsGetDetails"/>
   <int value="3398" label="DigitalGoodsAcknowledge"/>
+  <int value="3399" label="MediaRecorder_MimeType"/>
+  <int value="3400" label="MediaRecorder_VideoBitsPerSecond"/>
+  <int value="3401" label="MediaRecorder_AudioBitsPerSecond"/>
 </enum>
 
 <enum name="FeaturePolicyAllowlistType">