cc: Add UMA for tracking decode duration for out of raster decodes.

cc/tiles/checker_image_tracker.cc

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/tiles/checker_image_tracker.h"
 
 #include "base/bind.h"
 #include "base/memory/ptr_util.h"
 #include "base/stl_util.h"
 #include "base/trace_event/trace_event.h"
@@ skipping 193 matching lines @@
   }
 
   PaintImage::Id image_id = outstanding_image_decode_.value().stable_id();
   DCHECK_EQ(image_id_to_decode_.count(image_id), 0u);
   TRACE_EVENT_ASYNC_BEGIN0("cc", "CheckerImageTracker::DeferImageDecode",
                            image_id);
   ImageController::ImageDecodeRequestId request_id =
       image_controller_->QueueImageDecode(
           outstanding_image_decode_.value().sk_image(),
           base::Bind(&CheckerImageTracker::DidFinishImageDecode,
-                     weak_factory_.GetWeakPtr(), image_id));
+                     weak_factory_.GetWeakPtr(), image_id),
+          ImageDecodeCache::ImageDecodeType::kCheckerImaging);

[vmpstr, 2017/05/30 20:15:12]

This is a bit awkward. This means that to use this interface, you need to
qualify the caller as one of the available ImageDecodeTypes. 

What's the reason for distinguishing between say checker imaging and js decode?
Can we just say that one source of images comes from the image decode queue, and
the other one comes from raster?

In other words, I'm not sure what you would be able to infer from checker
imaging times vs js decode times (they use the same method of decoding)

[Khushal, 2017/05/30 21:30:32]

Since we use heuristics to decide whether images should be checkered or not, I
wanted to see that we generally checker only on long decodes. And whether that
logic needs more fine-tuning. Do you think that's unnecessary?

 
   image_id_to_decode_.emplace(image_id, base::MakeUnique<ScopedDecodeHolder>(
                                             image_controller_, request_id));
 }
 
 }  // namespace cc