trace_processor: initial definition of columnar trace class

This class will form the common point which other classes will read
from/write to. It will be shared across the three virtual tables (event, thread
and process) and be read from.

Bug: 80416541
Change-Id: I9d431a98c2f4d7359338d55a03af09a04c01859e
diff --git a/BUILD.gn b/BUILD.gn
index 203b6fe..3faa1f1 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -106,6 +106,9 @@
       "tools/sanitizers_unittests",
     ]
   }
+  if (build_standalone) {
+    deps += [ "src/trace_processor:unittests" ]
+  }
 }
 
 if (!build_with_chromium) {