metrics: Create Template for Sharing Metrics Data Between Processes

Create a template that contains metrics data and is guarded by a lock
so that the metrics data can be shared between processes.

Bug: 1151343
Change-Id: Ic365f837e385ac984f47d883f3f5104702d2ed35
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2644749
Commit-Queue: weiliangc <weiliangc@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Steve Kobes <skobes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#848183}

diff --git a/cc/BUILD.gn b/cc/BUILD.gn
index 8c8f34b..9c44e43e 100644
--- a/cc/BUILD.gn
+++ b/cc/BUILD.gn

@@ -192,6 +192,7 @@
     "metrics/latency_ukm_reporter.h",
     "metrics/lcd_text_metrics_reporter.cc",
     "metrics/lcd_text_metrics_reporter.h",
+    "metrics/shared_metrics_buffer.h",
     "metrics/throughput_ukm_reporter.cc",
     "metrics/throughput_ukm_reporter.h",
     "metrics/total_frame_counter.cc",