Optimize CSS box-shadow performance by caching the SkMask of the blur effect.

src/effects/SkBlurMask.cpp

Index: src/effects/SkBlurMask.cpp
diff --git a/src/effects/SkBlurMask.cpp b/src/effects/SkBlurMask.cpp
index bf50845ab6cb1d308562ea44fa279133bcafd46f..d03ca7716ccab358919be85f9499e8d35e9d12c1 100644
--- a/src/effects/SkBlurMask.cpp
+++ b/src/effects/SkBlurMask.cpp
@@ -541,8 +541,13 @@ bool SkBlurMask::BoxBlur(SkMask* dst, const SkMask& src,
         int             sw = src.fBounds.width();
         int             sh = src.fBounds.height();
         const uint8_t*  sp = src.fImage;
-        uint8_t*        dp = SkMask::AllocImage(dstSize);
-        SkAutoTCallVProc<uint8_t, SkMask_FreeImage> autoCall(dp);
+        uint8_t* dp = NULL;
+        SkDiscardableMemory* dm = NULL;
+        if (dst->fUseDiscardableMemory && (dm = SkDiscardableMemory::Create(dstSize))) {
+            dp = static_cast<uint8_t*>(dm->data());
+            dst->fDiscardableMemory = dm;
+        } else
+            dp = SkMask::AllocImage(dstSize);
 
         // build the blurry destination
         SkAutoTMalloc<uint8_t>  tmpBuffer(dstSize);
@@ -588,6 +593,12 @@ bool SkBlurMask::BoxBlur(SkMask* dst, const SkMask& src,
             // now we allocate the "real" dst, mirror the size of src
             size_t srcSize = src.computeImageSize();
             if (0 == srcSize) {
+                if (dm) {
+                    dm->unlock();
+                    SkDELETE(dm);
+                } else if (dp) {
+                    SkMask::FreeImage(dp);
+                }
                 return false;   // too big to allocate, abort
             }
             dst->fImage = SkMask::AllocImage(srcSize);
@@ -595,12 +606,16 @@ bool SkBlurMask::BoxBlur(SkMask* dst, const SkMask& src,
                                 sp, src.fRowBytes,
                                 dp + passCount * (rx + ry * dst->fRowBytes),
                                 dst->fRowBytes, sw, sh);
-            SkMask::FreeImage(dp);
+            if (dm) {
+                dm->unlock();
+                SkDELETE(dm);
+            } else if (dp) {
+                SkMask::FreeImage(dp);
+            }
         } else if (style != kNormal_SkBlurStyle) {
             clamp_with_orig(dp + passCount * (rx + ry * dst->fRowBytes),
                             dst->fRowBytes, sp, src.fRowBytes, sw, sh, style);
         }
-        (void)autoCall.detach();
     }
 
     if (style == kInner_SkBlurStyle) {