Treat calls to CanvasRenderingContext2D.clearRect as operations that clear the canvas

Source/platform/graphics/skia/OpaqueRegionSkia.cpp

Index: Source/platform/graphics/skia/OpaqueRegionSkia.cpp
diff --git a/Source/platform/graphics/skia/OpaqueRegionSkia.cpp b/Source/platform/graphics/skia/OpaqueRegionSkia.cpp
index 24c0369d2ee641c597367d4f4541d6d0d6364fa1..dfcfce2ac872c4f137ba43a1c1e5aa41ffc982ae 100644
--- a/Source/platform/graphics/skia/OpaqueRegionSkia.cpp
+++ b/Source/platform/graphics/skia/OpaqueRegionSkia.cpp
@@ -41,6 +41,7 @@ namespace blink {
 
 OpaqueRegionSkia::OpaqueRegionSkia()
     : m_opaqueRect(SkRect::MakeEmpty())
+    , m_treatOverwriteAsOpaque(false)
 {
 }
 
@@ -96,6 +97,23 @@ static inline bool xfermodeIsOpaque(const SkPaint& paint, bool srcIsOpaque)
     }
 }
 
+static inline bool xfermodeIsOverwrite(const SkPaint& paint)
+{
+    SkXfermode* xfermode = paint.getXfermode();
+    if (!xfermode)
+        return false; // default to kSrcOver_Mode
+    SkXfermode::Mode mode;
+    if (!xfermode->asMode(&mode))
+        return false;
+    switch (mode) {
+    case SkXfermode::kSrc_Mode:
+    case SkXfermode::kClear_Mode:
+        return true;
+    default:
+        return false;
+    }
+}
+
 // Returns true if the xfermode will keep the dst opaque, assuming the dst is already opaque.
 static inline bool xfermodePreservesOpaque(const SkPaint& paint, bool srcIsOpaque)
 {
@@ -302,14 +320,21 @@ void OpaqueRegionSkia::didDraw(const GraphicsContext* context, const SkRect& rec
     else if (!targetRect.intersect(deviceClipRect))
         return;
 
+    if (m_treatOverwriteAsOpaque && fillsBounds && xfermodeIsOverwrite(paint)) {
+        markRectAsOpaque(targetRect);
+        return;
+    }
+
     bool drawsOpaque = paintIsOpaque(paint, drawType, sourceBitmap);
     bool xfersOpaque = xfermodeIsOpaque(paint, drawsOpaque);
-    bool preservesOpaque = xfermodePreservesOpaque(paint, drawsOpaque);
 
-    if (fillsBounds && xfersOpaque)
+    if (fillsBounds && xfersOpaque) {
         markRectAsOpaque(targetRect);
-    else if (!preservesOpaque)
-        markRectAsNonOpaque(targetRect);
+    } else if (!m_treatOverwriteAsOpaque) {
+        bool preservesOpaque = xfermodePreservesOpaque(paint, drawsOpaque);
+        if (!preservesOpaque)

[Stephen White, 2014/07/25 19:29:06]

Nit: unless I'm missing something, this could all be inline:

if (!m_treatOverwriteAsOpaque && !xfermodePreservesOpaque(paint, drawsOpaque)) {
    markRectAsNonOpaque(targetRect);
}

+            markRectAsNonOpaque(targetRect);
+    }
 }
 
 void OpaqueRegionSkia::didDrawUnbounded(const GraphicsContext* context, const SkPaint& paint, DrawType drawType)