Fix SkBitmapOperations::CreateDropShadow drawing too large a shadow.

This CL fixes an issue where shadows were larger than
ShadowValue::GetMargin(). This caused certain shadows to clip to the
view boundaries which looks bad.

BUG=489533

Review URL: https://codereview.chromium.org/1123393007

Cr-Commit-Position: refs/heads/master@{#330912}

ui/gfx/skbitmap_operations.cc

@@ -743,9 +743,11 @@ SkBitmap SkBitmapOperations::CreateDropShadow(
     SkBitmap shadow_image = SkBitmapOperations::CreateColorMask(bitmap,
                                                                 shadow.color());
 
+    // The blur is halved to produce a shadow that correctly fits within the
+    // |shadow_margin|.
+    SkScalar sigma = SkDoubleToScalar(shadow.blur() / 2);
     skia::RefPtr<SkBlurImageFilter> filter =
-        skia::AdoptRef(SkBlurImageFilter::Create(
-            SkDoubleToScalar(shadow.blur()), SkDoubleToScalar(shadow.blur())));
+        skia::AdoptRef(SkBlurImageFilter::Create(sigma, sigma));
     paint.setImageFilter(filter.get());
 
     canvas.saveLayer(0, &paint);