Add unittest for I420Buffer::Rotate.
Add unittest for I420Buffer::Rotate.

Also change pointer to const ref for CheckCrop helper.

BUG=webrtc:5682

Review-Url: https://codereview.webrtc.org/2406463002
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#14663}

TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc