TrackEvent: Rename track event context for readability

This patch renames TrackEventContext and its track event accessor to
just EventContext to make trace points a little more readable.

Bug: 132678367
Change-Id: I198617c2c8ac4e4b11f7d76b01d841f328eafd50
diff --git a/BUILD b/BUILD
index 2f67f25..05b06ed 100644
--- a/BUILD
+++ b/BUILD
@@ -410,6 +410,7 @@
     srcs = [
         "include/perfetto/tracing/buffer_exhausted_policy.h",
         "include/perfetto/tracing/data_source.h",
+        "include/perfetto/tracing/event_context.h",
         "include/perfetto/tracing/internal/basic_types.h",
         "include/perfetto/tracing/internal/data_source_internal.h",
         "include/perfetto/tracing/internal/tracing_muxer.h",
@@ -424,7 +425,6 @@
         "include/perfetto/tracing/tracing_backend.h",
         "include/perfetto/tracing/track_event.h",
         "include/perfetto/tracing/track_event_category_registry.h",
-        "include/perfetto/tracing/track_event_context.h",
         "include/perfetto/tracing/track_event_interned_data_index.h",
     ],
 )
@@ -1001,6 +1001,7 @@
     name = "src_tracing_client_api",
     srcs = [
         "src/tracing/data_source.cc",
+        "src/tracing/event_context.cc",
         "src/tracing/internal/in_process_tracing_backend.cc",
         "src/tracing/internal/in_process_tracing_backend.h",
         "src/tracing/internal/system_tracing_backend.cc",
@@ -1011,7 +1012,6 @@
         "src/tracing/platform.cc",
         "src/tracing/tracing.cc",
         "src/tracing/track_event_category_registry.cc",
-        "src/tracing/track_event_context.cc",
         "src/tracing/virtual_destructors.cc",
     ],
 )