Revert of Revert of Add SkBitmap::readPixels() and reimplement copyTo and SkCanvas::readPixels

Revert of Revert of Add SkBitmap::readPixels() and reimplement copyTo and SkCanvas::readPixels (https://codereview.chromium.org/382543005/)

Reason for revert:
found and fixed the asserts -- we were drawing with unpremul bitmaps, but our blits assumed they were premul. Fix was to detect this and just draw nothing.

Original issue's description:
> Revert of Add SkBitmap::readPixels() and reimplement copyTo and SkCanvas::readPixels (https://codereview.chromium.org/388803007/)
> 
> Reason for revert:
> still failing (randomly?) bench sometimes. need stack dump to diagnose.
> 
> Original issue's description:
> > Add SkBitmap::readPixels() and reimplement copyTo and SkCanvas::readPixels
> > usning it.
> > 
> > Revert "Revert of add readPixels() to SkBitmap (https://codereview.chromium.org/377303002/)"
> > 
> > This reverts commit d08cb905a7cc80d8fb868bbd14fffe1cd68adcce.
> > 
> > TBR=scroggo@google.com
> > 
> > Committed: https://skia.googlesource.com/skia/+/debba5c3d091159149f8a88ab5dcd44dd72e0dc7
> 
> TBR=reed@google.com
> NOTREECHECKS=true
> NOTRY=true
> 
> Committed: https://skia.googlesource.com/skia/+/651eaeadeb0b1407f5fe192aeda90db1680fa2b8

TBR=reed@google.com

[reed2, 2014-07-13 10:59:35]

Created Revert of Revert of Add SkBitmap::readPixels() and reimplement copyTo
and SkCanvas::readPixels

[reed2, 2014-07-13 10:59:57]

The CQ bit was unchecked by reed@chromium.org

[reed2, 2014-07-13 11:00:30]

The CQ bit was checked by reed@chromium.org

[commit-bot: I haz the power, 2014-07-13 11:01:11]

CQ is trying da patch. Follow status at
 https://skia-tree-status.appspot.com/cq/reed@chromium.org/392453002/1