[SVG] Shared <use> path geometry

Based on Fredrik Söderquist <fs@opera.com>'s idea.

For <use>-instanced SVGPathElements, reuse the referenced Path object (now
cached in SVGPath).  This ensures that all <path> instances paint using the
same SkPath, which in turn reduces the pressure on internal Skia path caches.

BUG=skia:4527,549242
R=fs@opera.com

Review URL: https://codereview.chromium.org/1425913004

Cr-Commit-Position: refs/heads/master@{#357138}

third_party/WebKit/Source/core/layout/svg/LayoutSVGShape.h

@@ -130,6 +130,9 @@ private:
 
 private:
     OwnPtr<AffineTransform> m_localTransform;
+    // TODO(fmalita): the Path is now cached in SVGPath; while this additional cache is just a
+    // shallow copy, it certainly has a complexity/state management cost (plus allocation & storage
+    // overhead) - so we should look into removing it.
     OwnPtr<Path> m_path;
     mutable OwnPtr<LayoutSVGShapeRareData> m_rareData;
 

third_party/WebKit/Source/core/svg/SVGElement.cpp

@@ -542,7 +542,7 @@ void SVGElement::cursorImageValueRemoved()
 }
 #endif
 
-SVGElement* SVGElement::correspondingElement()
+SVGElement* SVGElement::correspondingElement() const
 {
     ASSERT(!hasSVGRareData() || !svgRareData()->correspondingElement() || containingShadowRoot());
     return hasSVGRareData() ? svgRareData()->correspondingElement() : 0;

third_party/WebKit/Source/core/svg/SVGElement.h

@@ -124,7 +124,7 @@ public:
     void cursorImageValueRemoved();
 #endif
 
-    SVGElement* correspondingElement();
+    SVGElement* correspondingElement() const;
     void setCorrespondingElement(SVGElement*);
     SVGUseElement* correspondingUseElement() const;
 

third_party/WebKit/Source/core/svg/SVGElementRareData.h

@@ -63,7 +63,7 @@ public:
     SVGCursorElement* cursorElement() const { return m_cursorElement; }
     void setCursorElement(SVGCursorElement* cursorElement) { m_cursorElement = cursorElement; }
 
-    SVGElement* correspondingElement() { return m_correspondingElement.get(); }
+    SVGElement* correspondingElement() const { return m_correspondingElement.get(); }
     void setCorrespondingElement(SVGElement* correspondingElement) { m_correspondingElement = correspondingElement; }
 
     CSSCursorImageValue* cursorImageValue() const { return m_cursorImageValue; }

third_party/WebKit/Source/core/svg/SVGPath.cpp

@@ -32,6 +32,7 @@
 #include "core/svg/SVGPathByteStreamSource.h"
 #include "core/svg/SVGPathParser.h"
 #include "core/svg/SVGPathUtilities.h"
+#include "platform/graphics/Path.h"
 
 namespace blink {
 
@@ -50,6 +51,16 @@ SVGPath::~SVGPath()
 {
 }
 
+const Path& SVGPath::path() const
+{
+    if (!m_cachedPath) {
+        m_cachedPath = adoptPtr(new Path);
+        buildPathFromByteStream(byteStream(), *m_cachedPath);
+    }
+
+    return *m_cachedPath;
+}
+
 PassRefPtrWillBeRawPtr<SVGPath> SVGPath::clone() const
 {
     return adoptRefWillBeNoop(new SVGPath(byteStream().copy()));
@@ -62,16 +73,28 @@ PassRefPtrWillBeRawPtr<SVGPropertyBase> SVGPath::cloneForAnimation(const String&
     return svgPath;
 }
 
+SVGPathByteStream& SVGPath::ensureByteStream()
+{
+    if (!m_byteStream)
+        m_byteStream = SVGPathByteStream::create();
+
+    return *m_byteStream.get();
+}
+
+void SVGPath::byteStreamWillChange()
+{
+    m_cachedPath.clear();
+}
+
 const SVGPathByteStream& SVGPath::byteStream() const
 {
-    return const_cast<SVGPath*>(this)->mutableByteStream();
+    return const_cast<SVGPath*>(this)->ensureByteStream();
 }
 
 SVGPathByteStream& SVGPath::mutableByteStream()
 {
-    if (!m_byteStream)
-        m_byteStream = SVGPathByteStream::create();
-    return *m_byteStream.get();
+    byteStreamWillChange();
+    return ensureByteStream();
 }
 
 String SVGPath::valueAsString() const
@@ -118,6 +141,8 @@ void SVGPath::calculateAnimatedValue(SVGAnimationElement* animationElement, floa
         fromStream = copy.get();
     }
 
+    byteStreamWillChange();
+
     // If the 'from' value is given and it's length doesn't match the 'to' value list length, fallback to a discrete animation.
     if (fromStream->size() != toStream.size() && fromStream->size()) {
         if (percentage < 0.5) {

third_party/WebKit/Source/core/svg/SVGPath.h

@@ -36,6 +36,7 @@
 namespace blink {
 
 class ExceptionState;
+class Path;
 class SVGPathByteStream;
 
 class SVGPath : public SVGPropertyBase {
@@ -49,6 +50,8 @@ public:
 
     ~SVGPath() override;
 
+    const Path& path() const;
+
     const SVGPathByteStream& byteStream() const;
     SVGPathByteStream& mutableByteStream();
 
@@ -68,7 +71,11 @@ private:
     SVGPath();
     explicit SVGPath(PassOwnPtr<SVGPathByteStream>);
 
+    SVGPathByteStream& ensureByteStream();
+    void byteStreamWillChange();
+
     OwnPtr<SVGPathByteStream> m_byteStream;
+    mutable OwnPtr<Path> m_cachedPath;
 };
 
 inline PassRefPtrWillBeRawPtr<SVGPath> toSVGPath(PassRefPtrWillBeRawPtr<SVGPropertyBase> passBase)

third_party/WebKit/Source/core/svg/SVGPathElement.cpp

@@ -72,9 +72,11 @@ DEFINE_NODE_FACTORY(SVGPathElement)
 
 Path SVGPathElement::asPath() const
 {
-    Path path;
-    buildPathFromByteStream(pathByteStream(), path);
-    return path;
+    // If this is a <use> instance, return the referenced path to maximize geometry sharing.
+    if (const SVGElement* element = correspondingElement())
+        return toSVGPathElement(element)->asPath();
+
+    return m_path->currentValue()->path();
 }
 
 float SVGPathElement::getTotalLength()