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-Commit-Position: refs/heads/master@{#14456}

webrtc/test/frame_utils.cc

@@ -72,5 +72,22 @@ bool FrameBufsEqual(const rtc::scoped_refptr<webrtc::VideoFrameBuffer>& f1,
                     half_width, half_height);
 }
 
+rtc::scoped_refptr<I420Buffer> ReadI420Buffer(int width, int height, FILE *f) {
+  int half_width = (width + 1) / 2;
+  rtc::scoped_refptr<I420Buffer> buffer(
+      // Explicit stride, no padding between rows.
+      I420Buffer::Create(width, height, width, half_width, half_width));
+  size_t size_y = static_cast<size_t>(width) * height;
+  size_t size_uv = static_cast<size_t>(half_width) * ((height + 1) / 2);
+
+  if (fread(buffer->MutableDataY(), 1, size_y, f) < size_y)
+    return nullptr;
+  if (fread(buffer->MutableDataU(), 1, size_uv, f) < size_uv)
+    return nullptr;
+  if (fread(buffer->MutableDataV(), 1, size_uv, f) < size_uv)
+    return nullptr;
+  return buffer;
+}
+
 }  // namespace test
 }  // namespace webrtc