Corrections of the render buffering scheme in AEC3 to ensure causality

This CL modifies the refactored render buffering scheme in AEC3
so that:
-A non-causal state can never occur which means that situations with
 nonrecoverable echo should not occur.
-For a stable audio pipeline with a predefined API call jitter,
 render overruns and underruns can never occur.

Bug: webrtc:8629,chromium:793305
Change-Id: I06ba1c368f92db95274090b08475dd02dbb85145
Reviewed-on: https://webrtc-review.googlesource.com/29861
Commit-Queue: Per Åhgren <peah@webrtc.org>
Reviewed-by: Gustaf Ullberg <gustaf@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21215}

modules/audio_processing/aec3/block_processor_unittest.cc

@@ -116,9 +116,6 @@ TEST(BlockProcessor, DISABLED_DelayControllerIntegration) {
     EXPECT_CALL(*render_delay_buffer_mock, Insert(_))
         .Times(kNumBlocks)
         .WillRepeatedly(Return(RenderDelayBuffer::BufferingEvent::kNone));
-    EXPECT_CALL(*render_delay_buffer_mock, IsBlockAvailable())
-        .Times(kNumBlocks)
-        .WillRepeatedly(Return(true));
     EXPECT_CALL(*render_delay_buffer_mock, SetDelay(kDelayInBlocks))
         .Times(AtLeast(1));
     EXPECT_CALL(*render_delay_buffer_mock, MaxDelay()).WillOnce(Return(30));
@@ -161,10 +158,7 @@ TEST(BlockProcessor, DISABLED_SubmoduleIntegration) {
     EXPECT_CALL(*render_delay_buffer_mock, Insert(_))
         .Times(kNumBlocks - 1)
         .WillRepeatedly(Return(RenderDelayBuffer::BufferingEvent::kNone));
-    EXPECT_CALL(*render_delay_buffer_mock, IsBlockAvailable())
-        .Times(kNumBlocks)
-        .WillRepeatedly(Return(true));
-    EXPECT_CALL(*render_delay_buffer_mock, PrepareCaptureCall())
+    EXPECT_CALL(*render_delay_buffer_mock, PrepareCaptureProcessing())
         .Times(kNumBlocks);
     EXPECT_CALL(*render_delay_buffer_mock, SetDelay(9)).Times(AtLeast(1));
     EXPECT_CALL(*render_delay_buffer_mock, Delay())