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This CL introduces the basic structure of Shape Detection API, including
four classes:
Detector, FaceDetector, DetectedObject, DetectedFace (dictionaries are
not yet implemented)

and adds simple layout tests in shapedetection-creation.html to verify
corresponding constructors.

R=mcasas@chromium.org
BUG=646083

Review-Url: https://codereview.chromium.org/2360783003
Cr-Commit-Position: refs/heads/master@{#421290}