add gm that exercises compose shader allocations

This gm triggers the assert in SkSmallAllocator.h
commented out by this CL.

PDFium constructs shaders that resemble the GM.

R=reed@google.com
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2249703002

Review-Url: https://codereview.chromium.org/2249703002