Adding optimization to avoid image copy in SkSurface copy on write when content is...

Adding optimization to avoid image copy in SkSurface copy on write when content is discardable

This patch also adds code to SkDeferredCanvas to trigger the optimization.

TEST=DeferredSurfaceCopy bench, Surface unit test
R=reed@google.com

Author: junov@chromium.org

Review URL: https://chromiumcodereview.appspot.com/14063015

git-svn-id: http://skia.googlecode.com/svn/trunk@8797 2bbb7eff-a529-9590-31e7-b0007b416f81