Harden degenerate gradient context handling

Certain inputs produce degenerate values at context creation time only.

Detect such cases after context creation, and abort drawing by returning
a null shader context instead.

BUG=skia:5821
R=reed@google.com,brianosman@google.com
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2397473003

Review-Url: https://codereview.chromium.org/2397473003
9 files changed