Expose IsSolidColor and write units tests

cc/resources/picture_pile.cc

Index: cc/resources/picture_pile.cc
diff --git a/cc/resources/picture_pile.cc b/cc/resources/picture_pile.cc
index 30375217b5825ab628c63861bb4ae1193294b87b..d4af28c73d3cc7e3762aaf9f592a3a0cbaf39740 100644
--- a/cc/resources/picture_pile.cc
+++ b/cc/resources/picture_pile.cc
@@ -146,7 +146,11 @@ float ClusterTiles(const std::vector<gfx::Rect>& invalid_tiles,
 
 namespace cc {
 
-PicturePile::PicturePile() : is_suitable_for_gpu_rasterization_(true) {}
+PicturePile::PicturePile()
+    : is_suitable_for_gpu_rasterization_(true),
+      is_solid_color_(true),
+      solid_color_(SK_ColorTRANSPARENT) {
+}
 
 PicturePile::~PicturePile() {
 }
@@ -393,6 +397,7 @@ bool PicturePile::UpdateAndExpandInvalidation(
         found_tile_for_recorded_picture = true;
       }
     }
+    DetermineIfSolidColor();
     DCHECK(found_tile_for_recorded_picture);
   }
 
@@ -408,4 +413,34 @@ void PicturePile::SetEmptyBounds() {
   recorded_viewport_ = gfx::Rect();
 }
 
+void PicturePile::DetermineIfSolidColor() {
+  is_solid_color_ = false;
+  solid_color_ = SK_ColorTRANSPARENT;
+
+  // determine if the entire picture map is pointing to the same image

[vmpstr, 2014/08/20 16:12:00]

style nit: Comments should be proper sentences (capitalization and a period at
the end)

[hendrikw, 2014/08/20 20:46:55]

Done.

+  if (picture_map_.empty()) {

[vmpstr, 2014/08/20 16:12:00]

We talked briefly about this yesterday: Is it possible that the picture_map_ is
empty? If not, this should be a DCHECK. I'm OK with handling it as well,
however. Up to you.

[hendrikw, 2014/08/20 20:46:55]

I think there are situations where the map could be empty.

+    is_solid_color_ = true;  // no pictures, we're solid 100% transparent
+    return;
+  }
+
+  const int kOpCountThatIsOkToAnalyze = 10;  // TODO(hendrikw) good value?

[vmpstr, 2014/08/20 16:12:00]

Looks like a good enough value. Can you put this in anonymous namespace near the
top of the file, please? Also as an FYI, typically we put TODOs above the line
:)

[hendrikw, 2014/08/20 20:46:55]

Done.

+  PictureMap::const_iterator it = picture_map_.begin();
+  Picture* picture = it->second.GetPicture();
+  DCHECK(picture);
+
+  // don't bother doing more work if the first image is too complicated
+  if (picture->GetApproximateOpCount() > kOpCountThatIsOkToAnalyze)
+    return;
+
+  // make sure all of the mapped images point to the same picture
+  while (++it != picture_map_.end()) {

[vmpstr, 2014/08/20 16:12:00]

I'd still prefer a for loop here:

for (++it; it != picture_map_.end(); ++it) {
 ...
}

[hendrikw, 2014/08/20 20:46:55]

Done.

+    if (it->second.GetPicture() != picture)
+      return;
+  }
+  skia::AnalysisCanvas canvas(recorded_viewport_.width(),
+                              recorded_viewport_.height());
+  picture->RasterForAnalysis(&canvas);
+  is_solid_color_ = canvas.GetColorIfSolid(&solid_color_);
+}
+
 }  // namespace cc