Do not clear canvas with transparent color after creating SkSurface

Recently SkSurface::MakeRaster changed its implementation to zero its
malloced space. So after creating the SkSurface, there is no need to
call sksurface's canvas->clear(SK_ColorTRANSPARENT). This CL removes
that step.

Review-Url: https://codereview.chromium.org/2119623003
Cr-Commit-Position: refs/heads/master@{#403907}

third_party/WebKit/Source/core/html/HTMLCanvasElement.cpp

@@ -132,7 +132,6 @@ PassRefPtr<Image> createTransparentImage(const IntSize& size)
 {
     DCHECK(canCreateImageBuffer(size));
     sk_sp<SkSurface> surface = SkSurface::MakeRasterN32Premul(size.width(), size.height());
-    surface->getCanvas()->clear(SK_ColorTRANSPARENT);
     return StaticBitmapImage::create(fromSkSp(surface->makeImageSnapshot()));
 }
 

third_party/WebKit/Source/platform/DragImage.cpp

@@ -107,7 +107,6 @@ PassRefPtr<SkImage> DragImage::resizeAndOrientImage(PassRefPtr<SkImage> image, I
         ? kNone_SkFilterQuality : kHigh_SkFilterQuality);
 
     SkCanvas* canvas = surface->getCanvas();
-    canvas->clear(SK_ColorTRANSPARENT);
     canvas->concat(affineTransformToSkMatrix(transform));
     canvas->drawImage(image.get(), 0, 0, &paint);
 

third_party/WebKit/Source/platform/graphics/ImagePattern.cpp

@@ -60,7 +60,6 @@ sk_sp<SkShader> ImagePattern::createShader(const SkMatrix& localMatrix) const
     if (!surface)
         return SkShader::MakeColorShader(SK_ColorTRANSPARENT);
 
-    surface->getCanvas()->clear(SK_ColorTRANSPARENT);
     SkPaint paint;
     paint.setXfermodeMode(SkXfermode::kSrc_Mode);
     surface->getCanvas()->drawImage(m_tileImage, 0, 0, &paint);