Throttle picture raster cache (flutter/engine#7759)

This decreases worst_frame_rasterizer_time_millis from 30ms to 10ms when
we enabled picture raster cache in tiles_scroll (i.e., lower the
threshold from 10 to 5).

engine/src/flutter/flow/raster_cache.cc

@@ -36,8 +36,11 @@ void RasterCacheResult::draw(SkCanvas& canvas, const SkPaint* paint) const {
   canvas.drawImage(image_, bounds.fLeft, bounds.fTop, paint);
 }
 
-RasterCache::RasterCache(size_t threshold)
-    : threshold_(threshold), checkerboard_images_(false), weak_factory_(this) {}
+RasterCache::RasterCache(size_t threshold, size_t picture_cache_limit_per_frame)
+    : threshold_(threshold),
+      picture_cache_limit_per_frame_(picture_cache_limit_per_frame),
+      checkerboard_images_(false),
+      weak_factory_(this) {}
 
 RasterCache::~RasterCache() = default;
 
@@ -182,6 +185,9 @@ bool RasterCache::Prepare(GrContext* context,
                           SkColorSpace* dst_color_space,
                           bool is_complex,
                           bool will_change) {
+  if (picture_cached_this_frame_ >= picture_cache_limit_per_frame_) {
+    return false;
+  }
   if (!IsPictureWorthRasterizing(picture, will_change, is_complex)) {
     // We only deal with pictures that are worthy of rasterization.
     return false;
@@ -211,6 +217,7 @@ bool RasterCache::Prepare(GrContext* context,
     entry.image = RasterizePicture(picture, context, transformation_matrix,
                                    dst_color_space, checkerboard_images_);
   }
+  picture_cached_this_frame_++;
   return true;
 }
 
@@ -232,6 +239,7 @@ void RasterCache::SweepAfterFrame() {
   using LayerCache = LayerRasterCacheKey::Map<Entry>;
   SweepOneCacheAfterFrame<PictureCache, PictureCache::iterator>(picture_cache_);
   SweepOneCacheAfterFrame<LayerCache, LayerCache::iterator>(layer_cache_);
+  picture_cached_this_frame_ = 0;
 }
 
 void RasterCache::Clear() {

engine/src/flutter/flow/raster_cache.h

@@ -42,7 +42,15 @@ struct PrerollContext;
 
 class RasterCache {
  public:
-  explicit RasterCache(size_t threshold = 3);
+  // The default max number of picture raster caches to be generated per frame.
+  // Generating too many caches in one frame may cause jank on that frame. This
+  // limit allows us to throttle the cache and distribute the work across
+  // multiple frames.
+  static constexpr int kDefaultPictureCacheLimitPerFrame = 3;
+
+  explicit RasterCache(
+      size_t threshold = 3,
+      size_t picture_cache_limit_per_frame = kDefaultPictureCacheLimitPerFrame);
 
   ~RasterCache();
 
@@ -67,6 +75,8 @@ class RasterCache {
   // 1. The picture is not worth rasterizing
   // 2. The matrix is singular
   // 3. The picture is accessed too few times
+  // 4. There are too many pictures to be cached in the current frame.
+  //    (See also kDefaultPictureCacheLimitPerFrame.)
   bool Prepare(GrContext* context,
                SkPicture* picture,
                const SkMatrix& transformation_matrix,
@@ -110,6 +120,8 @@ class RasterCache {
   }
 
   const size_t threshold_;
+  const size_t picture_cache_limit_per_frame_;
+  size_t picture_cached_this_frame_ = 0;
   PictureRasterCacheKey::Map<Entry> picture_cache_;
   LayerRasterCacheKey::Map<Entry> layer_cache_;
   bool checkerboard_images_;