Add cheapness predictor success histogram

cc/tile_manager.cc

Index: cc/tile_manager.cc
diff --git a/cc/tile_manager.cc b/cc/tile_manager.cc
index 2a426d35608af31d4ae3e9fcd7ff1bd4c3740e20..fa54ebdfcd963de1d8bc3cc96e92fa4119a92acf 100644
--- a/cc/tile_manager.cc
+++ b/cc/tile_manager.cc
@@ -657,6 +657,7 @@ void TileManager::DispatchOneRasterTask(scoped_refptr<Tile> tile) {
                       resource_id),
                   tile->content_rect_,
                   tile->contents_scale(),
+                  use_cheapness_estimator_,
                   tile->picture_pile(),
                   &rendering_stats_);
     TileManager::OnRasterTaskCompleted(tile, resource.Pass(),
@@ -670,7 +671,8 @@ void TileManager::DispatchOneRasterTask(scoped_refptr<Tile> tile) {
                  resource_pool_->resource_provider()->mapPixelBuffer(
                      resource_id),
                  tile->content_rect_,
-                 tile->contents_scale()),
+                 tile->contents_scale(),
+                 use_cheapness_estimator_),
       base::Bind(&TileManager::OnRasterTaskCompletedByWorker,
                  base::Unretained(this),
                  tile,
@@ -776,6 +778,7 @@ void TileManager::DidTileBinChange(Tile* tile,
 void TileManager::RunRasterTask(uint8* buffer,
                                 const gfx::Rect& rect,
                                 float contents_scale,
+                                bool use_cheapness_estimator,
                                 PicturePileImpl* picture_pile,
                                 RenderingStats* stats) {
   TRACE_EVENT0("cc", "TileManager::RunRasterTask");
@@ -802,6 +805,21 @@ void TileManager::RunRasterTask(uint8* buffer,
       duration.InMilliseconds(),
       0, 10, 10
     );
+
+    if (use_cheapness_estimator) {
+      bool is_cheap = picture_pile->IsCheapInRect (rect, contents_scale);
+      bool is_correct_estimate = false;
+      
+      // Cheap means less than 1ms
+      if ((is_cheap && duration.InMillisecondsF() <= 1.0f) ||
+          (!is_cheap && duration.InMillisecondsF() > 1.0f))
+        is_correct_estimate = true;
+
+      UMA_HISTOGRAM_BOOLEAN(
+        "Renderer4.CheapnessPredictorAccuracy",
+        is_correct_estimate
+      );
+    }
   }
 }