Implement basic parts of hit regions on canvas2d.

Source/platform/graphics/GraphicsContext.cpp

Index: Source/platform/graphics/GraphicsContext.cpp
diff --git a/Source/platform/graphics/GraphicsContext.cpp b/Source/platform/graphics/GraphicsContext.cpp
index a77513527099b7b3682f774ac39d35461809701b..5d01475acd93a7a59ffb0cb7f8103a5079142925 100644
--- a/Source/platform/graphics/GraphicsContext.cpp
+++ b/Source/platform/graphics/GraphicsContext.cpp
@@ -49,6 +49,7 @@
 #include "third_party/skia/include/effects/SkLumaColorFilter.h"
 #include "third_party/skia/include/gpu/GrRenderTarget.h"
 #include "third_party/skia/include/gpu/GrTexture.h"
+#include "third_party/skia/include/pathops/SkPathOps.h"
 #include "wtf/Assertions.h"
 #include "wtf/MathExtras.h"
 
@@ -1377,6 +1378,34 @@ void GraphicsContext::clipPath(const Path& pathToClip, WindRule clipRule)
     path.setFillType(previousFillType);
 }
 
+Path GraphicsContext::getCurrentClipPath(const Path& path)
+{
+    if (contextDisabled())
+        return Path();
+
+    const SkClipStack* clipStack = m_canvas->getClipStack();
+    SkClipStack::Iter iter(*clipStack, SkClipStack::Iter::kBottom_IterStart);
+    const SkClipStack::Element* element;
+
+    SkPath totalClipPath(path.skPath());

[fs, 2014/06/09 11:06:27]

This doesn't appear to work in the general case - i.e. consider for instance a
sequence of an empty element followed by a non-empty replace-op element (or
really just a replace-op element). That may or may not be problematic in 
practice, but it'd probably make sense to at least document (and possibly
assert) what isn't expected to work.

[zino, 2014/06/10 10:09:34]

Done.

+
+    while ((element = iter.next())) {
+
+        SkClipStack::Element::Type type = element->getType();
+        SkPath clipPath;
+        if (type != SkClipStack::Element::kEmpty_Type)
+            element->asPath(&clipPath);
+
+        SkRegion::Op elementOp = element->getOp();
+        if (elementOp == SkRegion::kReplace_Op)
+            totalClipPath = clipPath;

[fs, 2014/06/09 11:06:27]

Since the result of this method should be the intersection between |path| and
the clip-stack, this won't do the right thing. Maybe it'd be better to simply
compute the actual clip-path, and then perform the intersection against |path|?

[zino, 2014/06/10 10:09:34]

Done.

+        else
+            Op(totalClipPath, clipPath, (SkPathOp)element->getOp(), &totalClipPath);
+    }
+
+    return Path(totalClipPath);
+}
+
 void GraphicsContext::clipConvexPolygon(size_t numPoints, const FloatPoint* points, bool antialiased)
 {
     if (contextDisabled())