Revert of Remove SK_SUPPORT_LEGACY_DEEPFLATTENING. (patchset #1 id:1 of https://codereview.chromium.org/769953002/)

Reason for revert:
Breaks canary builds.  Will reland after the Chromium change lands.

Original issue's description:
> Remove SK_SUPPORT_LEGACY_DEEPFLATTENING.
>
> This was needed for pictures before v33, and we're now requiring v35+.
>
> Will follow up with the same for skia/ext/pixel_ref_utils_unittest.cc
>
> BUG=skia:
>
> Committed: https://skia.googlesource.com/skia/+/52c293547b973f7fb5de3c83f5062b07d759ab88

TBR=reed@google.com,mtklein@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=skia:

Review URL: https://codereview.chromium.org/768183002
diff --git a/src/effects/SkLayerRasterizer.cpp b/src/effects/SkLayerRasterizer.cpp
index d3e112c..b331a03 100644
--- a/src/effects/SkLayerRasterizer.cpp
+++ b/src/effects/SkLayerRasterizer.cpp
@@ -148,13 +148,18 @@
     return true;
 }
 
+#ifdef SK_SUPPORT_LEGACY_DEEPFLATTENING
+SkLayerRasterizer::SkLayerRasterizer(SkReadBuffer& buffer)
+    : SkRasterizer(buffer), fLayers(ReadLayers(buffer)) {}
+#endif
+
 SkFlattenable* SkLayerRasterizer::CreateProc(SkReadBuffer& buffer) {
     return SkNEW_ARGS(SkLayerRasterizer, (ReadLayers(buffer)));
 }
 
 SkDeque* SkLayerRasterizer::ReadLayers(SkReadBuffer& buffer) {
     int count = buffer.readInt();
-
+    
     SkDeque* layers = SkNEW_ARGS(SkDeque, (sizeof(SkLayerRasterizer_Rec)));
     for (int i = 0; i < count; i++) {
         SkLayerRasterizer_Rec* rec = (SkLayerRasterizer_Rec*)layers->push_back();