Verify the full size returned by PIEX

cherry-pick of d0e0885d90cecd43b717d89b565d9a713cffd90c in master

Original commit message below:

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Verify the full size returned by PIEX

As the full size info is only optional, one needs to verify the values
before using them

BUG=b/27422694
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1757273002

Review URL: https://codereview.chromium.org/1757273002
NOTREECHECKS=true
NOTRY=true
NOPRESUBMIT=true

Review URL: https://codereview.chromium.org/1757353002
1 file changed