remove call to deprecated preroll() -- cc manages triggering/caching decodes

I just now (after posting this) see https://bugs.chromium.org/p/chromium/issues/detail?id=595090

Calling preroll() is (1) deprecated, and (2) is no guarantee that the bits will hang around, since the cache is volatile. If the desire is to "force" the codec to produce a raster, then lest just ask the codec to do that (i.e. *not* be lazy).

thoughts?

BUG=

Review-Url: https://codereview.chromium.org/2587433003
Cr-Commit-Position: refs/heads/master@{#439878}

third_party/WebKit/Source/core/frame/ImageBitmap.cpp

@@ -506,8 +506,6 @@ static PassRefPtr<StaticBitmapImage> cropImageAndApplyColorSpaceConversion(
     if (parsedOptions.premultiplyAlpha && imageFormat == DontPremultiplyAlpha)
       return StaticBitmapImage::create(
           unPremulSkImageToPremul(croppedSkImage.get()));
-    // Call preroll to trigger image decoding.
-    croppedSkImage->preroll();
     return StaticBitmapImage::create(std::move(croppedSkImage));
   }