document the SkStrAppend methods

BUG=skia:

Review URL: https://codereview.chromium.org/709063002
Rebaseline for https://codereview.chromium.org/703463002/.

TBR=egdaniel@google.com

Review URL: https://codereview.chromium.org/709043002
Refactor DrawTarget and GPU to be independent

BUG=skia:

Review URL: https://codereview.chromium.org/705593002
Missed some expectations

TBR=egdaniel@google.com

Review URL: https://codereview.chromium.org/705293002