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.

R=reed@chromium.org
TBR=scroggo@google.com

Author: reed@google.com

Review URL: https://codereview.chromium.org/388803007
7 files changed