Use BBH reserve hook to preallocate space for tiles.

src/core/SkTileGrid.cpp

Index: src/core/SkTileGrid.cpp
diff --git a/src/core/SkTileGrid.cpp b/src/core/SkTileGrid.cpp
index 8b96b73fa13f895654c53d7834d28451479bd163..30ca4b9179b7e29d71425a65b0ad4d1dfd31fc8b 100644
--- a/src/core/SkTileGrid.cpp
+++ b/src/core/SkTileGrid.cpp
@@ -23,6 +23,33 @@ SkTileGrid::~SkTileGrid() {
     SkDELETE_ARRAY(fTiles);
 }
 
+void SkTileGrid::reserve(unsigned opCount) {
+    if (fXTiles * fYTiles == 0) {
+        return;  // A tileless tile grid is nonsensical, but happens in at least cc_unittests.
+    }
+
+    // If we assume every op we're about to try to insert() falls within our grid bounds,
+    // then every op has to hit at least one tile.  In fact, a quick scan over our small
+    // SKP set shows that in the average SKP, each op hits two 256x256 tiles.
+
+    // If we take those observations and further assume the ops are distributed evenly
+    // across the picture, we get this guess for number of ops per tile:
+    const int opsPerTileGuess = (2 * opCount) / (fXTiles * fYTiles);
+
+    for (SkTDArray<unsigned>* tile = fTiles; tile != fTiles + (fXTiles * fYTiles); tile++) {
+        tile->setReserve(opsPerTileGuess);
+    }
+
+    // In practice, this heuristic means we'll temporarily allocate about 30% more bytes
+    // than if we made no setReserve() calls, but time spent in insert() drops by about 50%.
+}
+
+void SkTileGrid::flushDeferredInserts() {
+    for (SkTDArray<unsigned>* tile = fTiles; tile != fTiles + (fXTiles * fYTiles); tile++) {
+        tile->shrinkToFit();
+    }
+}
+
 // Adjustments to user-provided bounds common to both insert() and search().
 // Call this after making insert- or search- specific adjustments.
 void SkTileGrid::commonAdjust(SkRect* rect) const {