Add new experimental API to SkPicture to get "id" of current op

Add new experimental API to SkPicture to get "id" of current op

When gathering information about a picture (in the new SkDevice::EXPERIMENTAL_optimize entry point) it is necessary to be able to correlate the gathered information with the command in the SkPicture (so the information can later be combined with the similarly indexed information from the BBH). This entry point exposes that information to friend classes.

R=reed@google.com

Author: robertphillips@google.com

Review URL: https://codereview.chromium.org/206853003

git-svn-id: http://skia.googlecode.com/svn/trunk@13919 2bbb7eff-a529-9590-31e7-b0007b416f81