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It appears that under certain conditions (ASAN), the |script_state_|
in the CSSPaintDefinition is nullptr. Notice that the |script_state_|
itself is a scroped_refptr, it being null indicates that the
CSSPaintDefinition object itself is null.

The cluster fuzz seems to be able to repro this with a 29KB minimized
test case and I can never repro it locally with the same build args.
This CL is a speculate fix, and let's wait for fuzzer to tell us whether
the problem is fixed by this or not.

Bug: 806082
Change-Id: Iad22be412709d697d42e111cbf74de972b094918
Reviewed-on: https://chromium-review.googlesource.com/891598Reviewed-by: Stephen McGruer <smcgruer@chromium.org>
Commit-Queue: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532932}