[throughput metrics] Accumulate metrics across sequences of same type.

If a sequence does not have enough frames, then instead of discarding
them completely, retain the throughput-data, and accumulate them with
the throughput metrics of the subsequent sequences (of the same
interaction-type) until there are enough frames to report the metric.

Bug: 1031251
Change-Id: I4bbe1d0b024e04afd92677426a546699d9cc85d7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1952529
Reviewed-by: Behdad Bakhshinategh <behdadb@chromium.org>
Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#722588}

diff --git a/cc/BUILD.gn b/cc/BUILD.gn
index 6e92c22..335cbec 100644
--- a/cc/BUILD.gn
+++ b/cc/BUILD.gn

@@ -645,6 +645,7 @@
     "metrics/compositor_frame_reporter_unittest.cc",
     "metrics/compositor_frame_reporting_controller_unittest.cc",
     "metrics/compositor_timing_history_unittest.cc",
+    "metrics/frame_sequence_metrics_unittest.cc",
     "metrics/frame_sequence_tracker_unittest.cc",
     "mojo_embedder/async_layer_tree_frame_sink_unittest.cc",
     "paint/discardable_image_map_unittest.cc",