Add FrameBufferProxy test for low-latency renderer

Ensures that frames are decoded instantly when in low-latency render
mode. This also tests the max queue size behaviour. Adds a new test
suite for FrameBufferProxy that sets the appropriate field trials.

* Fixes FrameDecodeTiming to never use negative wait times for decode
timestamps.

R=kron@webrtc.org

Change-Id: I06cbec52e1e866e21aa964b24c4fd0163c26961b
Bug: webrtc:13658
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/251601
Reviewed-by: Erik Språng <sprang@webrtc.org>
Reviewed-by: Johannes Kron <kron@webrtc.org>
Commit-Queue: Evan Shrubsole <eshr@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#35999}

video/frame_decode_timing_unittest.cc

diff --git a/video/frame_decode_timing_unittest.cc b/video/frame_decode_timing_unittest.cc
index 1932e85..ac5bf9e 100644
--- a/video/frame_decode_timing_unittest.cc
+++ b/video/frame_decode_timing_unittest.cc
@@ -89,7 +89,8 @@
 }
 
 TEST_F(FrameDecodeTimingTest, FastForwardsFrameTooFarInThePast) {
-  const TimeDelta decode_delay = TimeDelta::Millis(-6);
+  const TimeDelta decode_delay =
+      -FrameDecodeTiming::kMaxAllowedFrameDelay - TimeDelta::Millis(1);
   const Timestamp render_time = clock_.CurrentTime();
   timing_.SetTimes(90000, render_time, decode_delay);
 
@@ -99,14 +100,16 @@
 }
 
 TEST_F(FrameDecodeTimingTest, NoFastForwardIfOnlyFrameToDecode) {
-  const TimeDelta decode_delay = TimeDelta::Millis(-6);
+  const TimeDelta decode_delay =
+      -FrameDecodeTiming::kMaxAllowedFrameDelay - TimeDelta::Millis(1);
   const Timestamp render_time = clock_.CurrentTime();
   timing_.SetTimes(90000, render_time, decode_delay);
 
+  // Negative `decode_delay` means that `latest_decode_time` is now.
   EXPECT_THAT(frame_decode_scheduler_.OnFrameBufferUpdated(90000, 90000, false),
               Optional(AllOf(
                   Field(&FrameDecodeTiming::FrameSchedule::latest_decode_time,
-                        Eq(clock_.CurrentTime() + decode_delay)),
+                        Eq(clock_.CurrentTime())),
                   Field(&FrameDecodeTiming::FrameSchedule::render_time,
                         Eq(render_time)))));
 }