Trace processor: add trace_metadata table

Introduces a table to store trace metadata. The list of possible entries
currently consists of chrome benchmark metadata (like benchmark name,
start time, etc), but it's possible to add other items.

Bug: 132682848
Change-Id: Id5efc1ffbfb4a93ec7f9a4319055d1b018da1215
diff --git a/Android.bp b/Android.bp
index a8878fb..2cdffac 100644
--- a/Android.bp
+++ b/Android.bp
@@ -3244,6 +3244,7 @@
     "src/trace_processor/heap_profile_mapping_table.cc",
     "src/trace_processor/heap_profile_tracker.cc",
     "src/trace_processor/instants_table.cc",
+    "src/trace_processor/metadata_table.cc",
     "src/trace_processor/metrics/descriptors.cc",
     "src/trace_processor/metrics/metrics.cc",
     "src/trace_processor/process_table.cc",