Fix I420VideoFrame unittests
Fix I420VideoFrame unittests

* Only compare actual pixel data, not the padding between width and stride.
* When creating a frame from raw buffers with excessive size, do not assume that the frame’s allocated size will be as excessive as the input buffers.
* The arrays in TestI420VideoFrame.CopyFrame and TestI420VideoFrame.CloneFrame are too small, and we currently memcpy out of bounds.

I think this CL should land regardless, but the main purpose is to pave the way for for planned changes to I420VideoFrame. See https://review.webrtc.org/38879004.

R=stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/34199004

Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#8416}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 88828e77d9dd0d92848384bffccbc507ef8598d8

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