simplify uniqueID code in picture

patch from issue 750003002 at patchset 1 (http://crrev.com/750003002#ps1)

BUG=skia:

Review URL: https://codereview.chromium.org/752573002
add SkImage::newSurface

BUG=skia:

Review URL: https://codereview.chromium.org/741763002
All the small packing tweaks mentioned in the bug.

BUG=skia:3144

Review URL: https://codereview.chromium.org/748063002
remove GrAllocPool

BUG=skia:

Review URL: https://codereview.chromium.org/742253002