[tracing] Implement composable memory usage estimators.

base/BUILD.gn

Index: base/BUILD.gn
diff --git a/base/BUILD.gn b/base/BUILD.gn
index a3e3205eae9f137c15c774fdff736a88124201f8..d89ffdfe31a66a83a15ce66ccc3e0c7115b8924a 100644
--- a/base/BUILD.gn
+++ b/base/BUILD.gn
@@ -899,6 +899,7 @@ component("base") {
     "trace_event/blame_context.cc",
     "trace_event/blame_context.h",
     "trace_event/common/trace_event_common.h",
+    "trace_event/estimate_memory_usage.h",
     "trace_event/heap_profiler.h",
     "trace_event/heap_profiler_allocation_context.cc",
     "trace_event/heap_profiler_allocation_context.h",
@@ -2141,7 +2142,10 @@ test("base_unittests") {
   }
 
   if (use_experimental_allocator_shim) {
-    sources += [ "allocator/allocator_shim_unittest.cc" ]
+    sources += [
+      "allocator/allocator_shim_unittest.cc",
+      "trace_event/estimate_memory_usage_unittest.cc",
+    ]
   }
 
   # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.