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This CL homogenises the behaviour of the ShapeDetector's
detect() in case of an input element with any zero dimension.
Assuming such input element pass the cross origin test and
its underlying pixel data can be retrieved, then if either width
or height is zero, detect() Promise is resolved with an empty
array.

LayoutTests are adapted/extended/cleaned up.

BUG=674306

Review-Url: https://codereview.chromium.org/2626743002
Cr-Commit-Position: refs/heads/master@{#442737}