Vaapi decoder: use TRACE_COUNTER_ID2 for "Vaapi frames at client"

This CL addresses two issues:
- Extends "Vaapi frames at client" to show used vs available, where ToT
 only shows the currently used ones, without any reference to its max.
- TRACE_COUNTER1/2 is global per-process, which means that when there
 are multiple Vaapi decoders working, each will update the counter as if
 in isolation, rendering the measurement meaningless. This CL uses
 TRACE_COUNTER_ID2 to add an id to each counter, allowing for counter
 separation [1].

[1] https://i.imgur.com/qq60Xqw.png (https://imgur.com/a/AOcwh78)

Bug: 909926, 721674
Change-Id: If7089d8e9ca614912b3965eb46dc552b8c63928f
Reviewed-on: https://chromium-review.googlesource.com/c/1356884
Commit-Queue: Miguel Casas <mcasas@chromium.org>
Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612819}

media/gpu/vaapi/vaapi_video_decode_accelerator.cc

@@ -285,7 +285,9 @@ void VaapiVideoDecodeAccelerator::OutputPicture(
   }
   // Notify the client a picture is ready to be displayed.
   ++num_frames_at_client_;
-  TRACE_COUNTER1("media,gpu", "Vaapi frames at client", num_frames_at_client_);
+  TRACE_COUNTER_ID2("media,gpu", "Vaapi frames at client", this, "used",
+                    num_frames_at_client_, "available",
+                    pictures_.size() - num_frames_at_client_);
   DVLOGF(4) << "Notifying output picture id " << output_id << " for input "
             << input_id
             << " is ready. visible rect: " << visible_rect.ToString();
@@ -729,8 +731,9 @@ void VaapiVideoDecodeAccelerator::ReusePictureBuffer(
   }
 
   --num_frames_at_client_;
-  TRACE_COUNTER1("media,gpu", "Vaapi frames at client", num_frames_at_client_);
-
+  TRACE_COUNTER_ID2("media,gpu", "Vaapi frames at client", this, "used",
+                    num_frames_at_client_, "available",
+                    pictures_.size() - num_frames_at_client_);
   {
     base::AutoLock auto_lock(lock_);
     available_picture_buffers_.push_back(picture_buffer_id);