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 dfeba25829b62672785de68da86b914954ba14a8..e45d65c914842bd66314e2e7aa08b0183d67e035 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 same as

[jwd, 2017/01/16 21:03:07]

Nit: "is exactly the same as..." for all summaries.

+    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 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 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 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">
@@ -109611,6 +109633,8 @@ 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"

[Justin Novosad, 2017/01/13 20:09:54]

I don't see where this suffix is referenced.

[xlai (Olivia), 2017/01/13 20:13:11]

It's actually here:
https://cs.chromium.org/chromium/src/third_party/WebKit/Source/platform/image...

This is a function which records the image usage types for toDataURL, toBlob,
convertToBlob. I was checking up the usage data for convertToBlob today and
realize
that the histogram suffix is not present at all because the previous CL forgot 
to add one.

+      label="Image formats passed to OffscreenCanvas.convertToBlob (promise)"/>
   <affected-histogram name="Canvas.RequestedImageMimeTypes"/>
 </histogram_suffixes>