Complete usage tracking for OffscreenCanvas.convertToBlob API

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 6b52fd6c5aee75318a5bfc2e62dd574b23156816..7d4422f1b09f21de0c3d1d04cae4bf5dfd2dfcf6 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -4210,6 +4210,11 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
     in canvas.toBlob. When the encoding idle task is delayed for longer than
     IdleTaskCompleteTimeoutDelay, the browser will switch to a non-idle task to
     force encoding to happen on the main thread.
+
+    In addition, metric values from OffscreenCanvas.convertToBlob API call are
+    also gathered into this histogram, because the logic flow is exactly the
+    same as canvas.toBlob. It's worth to note that the values can come from idle
+    tasks on either main or worker thread.
   </summary>
 </histogram>
 
@@ -4218,6 +4223,11 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <owner>xlai@chromium.org</owner>
   <summary>
     Records the status of the idle task when finishing a toBlob call.
+
+    In addition, metric values from OffscreenCanvas.convertToBlob API call are
+    also gathered into this histogram, because the logic flow is exactly the
+    same as canvas.toBlob. It's worth to note that the values can come from idle
+    tasks on either main or worker thread.
   </summary>
 </histogram>
 
@@ -4234,13 +4244,25 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
     canvas.toBlob. When the initialization idle task is delayed for longer than
     IdleTaskStartTimeoutDelay, the browser will switch to a non-idle task to
     force initialization and encoding to occur on the main thread.
+
+    In addition, metric values from OffscreenCanvas.convertToBlob API call are
+    also gathered into this histogram, because the logic flow is exactly the
+    same as canvas.toBlob. It's worth to note that the values can come from idle
+    tasks on either main or worker thread.
   </summary>
 </histogram>
 
 <histogram name="Blink.Canvas.ToBlobDuration" units="microseconds">
   <owner>junov@chromium.org</owner>
   <owner>xlai@chromium.org</owner>
-  <summary>Time spent on 2D canvas toBlob API call.</summary>
+  <summary>
+    Time spent on 2D canvas toBlob API call.
+
+    In addition, metric values from OffscreenCanvas.convertToBlob API call are
+    also gathered into this histogram, because the logic flow is exactly the
+    same as canvas.toBlob. It's worth to note that the values can come from idle
+    tasks on either main or worker thread.
+  </summary>
 </histogram>
 
 <histogram name="Blink.Canvas.ToDataURL" units="microseconds">
@@ -109872,6 +109894,8 @@ value.
   <suffix name="toDataURL" label="Image formats passed to canvas.toDataURL"/>
   <suffix name="toBlobCallback"
       label="Image formats passed to canvas.toBlob (callback)"/>
+  <suffix name="convertToBlobPromise"
+      label="Image formats passed to OffscreenCanvas.convertToBlob (promise)"/>
   <affected-histogram name="Canvas.RequestedImageMimeTypes"/>
 </histogram_suffixes>