cc: Fix a bug where filter outsets for drop-shadow could be negative.

cc/output/filter_operations.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/output/filter_operations.h"
 
 #include <stddef.h>
 
 #include <cmath>
 #include <numeric>
@@ skipping 92 matching lines @@
     } else {
       if (op.type() == FilterOperation::BLUR ||
           op.type() == FilterOperation::DROP_SHADOW) {
         int spread = SpreadForStdDeviation(op.amount());
         if (op.type() == FilterOperation::BLUR) {
           *top += spread;
           *right += spread;
           *bottom += spread;
           *left += spread;
         } else {
-          *top += spread - op.drop_shadow_offset().y();
-          *right += spread + op.drop_shadow_offset().x();
-          *bottom += spread + op.drop_shadow_offset().y();
-          *left += spread - op.drop_shadow_offset().x();
+          *top += std::max(0, spread - op.drop_shadow_offset().y());

[enne (OOO), 2016/07/06 17:34:57]

What if your drop shadow is up and to the left instead of down and to the right?
Does (-3, -8) really have the same outsets as (3, 8)?

[jbroman, 2016/07/06 17:37:32]

I don't understand your question. No, it doesn't have the same outsets; it has
opposite outsets. For example, in the test below, the outsets for (-3, -8) would
be top=8, right=0, bottom=0, left=3.

This isn't std::abs(x); it's std::max(0, x). It only ensures that no edge gets
negative outsets.

[enne (OOO), 2016/07/06 17:57:18]

*drinks more coffee* Oops, sorry about misreading that.

+          *right += std::max(0, spread + op.drop_shadow_offset().x());
+          *bottom += std::max(0, spread + op.drop_shadow_offset().y());
+          *left += std::max(0, spread - op.drop_shadow_offset().x());
         }
       }
     }
   }
 }
 
 bool FilterOperations::HasFilterThatMovesPixels() const {
   for (size_t i = 0; i < operations_.size(); ++i) {
     const FilterOperation& op = operations_[i];
     switch (op.type()) {
@@ skipping 112 matching lines @@
 void FilterOperations::AsValueInto(
     base::trace_event::TracedValue* value) const {
   for (size_t i = 0; i < operations_.size(); ++i) {
     value->BeginDictionary();
     operations_[i].AsValueInto(value);
     value->EndDictionary();
   }
 }
 
 }  // namespace cc