replace etc1_pixel_ref with skia version (SkMallocPixelRef)

This is part of a larger effort to replace subclasses of pixelref in chrome/blink with equivalent skia versions, allowing faster iteration/evolution of bitmaps/pixelrefs in skia.

BUG=

Review URL: https://codereview.chromium.org/138393002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246469 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246469