[tracing] Expose interning helpers.

The source of truth for the interned data is tied to a particular
subclass of TrackEventInternedDataIndex.

Expose the implementations for event name / category / debug annotation
names to allow Chromium to use it.

R=eseckler@google.com,skyostil@google.com

Change-Id: I6fc7bb5d0f6c3f4f0358de8c6aca7509dfbf9449
diff --git a/BUILD b/BUILD
index 55074b2..55dcf56 100644
--- a/BUILD
+++ b/BUILD
@@ -528,6 +528,7 @@
         "include/perfetto/tracing/internal/tracing_tls.h",
         "include/perfetto/tracing/internal/track_event_data_source.h",
         "include/perfetto/tracing/internal/track_event_internal.h",
+        "include/perfetto/tracing/internal/track_event_interned_fields.h",
         "include/perfetto/tracing/internal/track_event_macros.h",
         "include/perfetto/tracing/internal/write_track_event_args.h",
         "include/perfetto/tracing/locked_handle.h",
@@ -1645,6 +1646,7 @@
         "src/tracing/internal/tracing_muxer_impl.cc",
         "src/tracing/internal/tracing_muxer_impl.h",
         "src/tracing/internal/track_event_internal.cc",
+        "src/tracing/internal/track_event_interned_fields.cc",
         "src/tracing/platform.cc",
         "src/tracing/traced_value.cc",
         "src/tracing/tracing.cc",