[cc/metrics] Track dropped-frames affecting smoothness separately.

Not all dropped-frames affect smoothness. For example, the compositor
thread may be driving a css animation, while the main-thread drops
frames while doing busy js work (e.g. handling event, etc.). These
dropped frames do not affectthe smoothness of the animation. So count
the dropped frames that specifically affect smoothness separately. To do
this, for each frame, track whether the compositor and/or the main
thread are affecting smoothness.

BUG=1115376

Change-Id: I6cd3e3d2a10ed63c53ab929386abae1d5e29e7bf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2366039
Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Khushal <khushalsagar@chromium.org>
Reviewed-by: Robert Flack <flackr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#802100}

diff --git a/cc/BUILD.gn b/cc/BUILD.gn
index 7e1189f..da22f50 100644
--- a/cc/BUILD.gn
+++ b/cc/BUILD.gn

@@ -679,6 +679,7 @@
     "metrics/compositor_frame_reporter_unittest.cc",
     "metrics/compositor_frame_reporting_controller_unittest.cc",
     "metrics/compositor_timing_history_unittest.cc",
+    "metrics/dropped_frame_counter_unittest.cc",
     "metrics/events_metrics_manager_unittest.cc",
     "metrics/frame_sequence_metrics_unittest.cc",
     "metrics/frame_sequence_tracker_unittest.cc",