New helper function test::ReadI420Buffer, refactor FrameReader to use it.

This change reduces the number of places where we first fread a I420
frame into a uint8_t buffer, followed by a copy into a frame buffer
object.

BUG=None

Review-Url: https://codereview.webrtc.org/2362683002
Cr-Original-Commit-Position: refs/heads/master@{#14456}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 115bd153c7f5a64eabbac222bad66e223abbfe2b
19 files changed