Fix format specifier for TracingBackendId (aka size_t)
Fix format specifier for TracingBackendId (aka size_t)

This fixes compile errors in Chrome when using startup tracing
via client library.

Change-Id: I4fd7b6afda1127eca47498f39524c4f992f877fe
google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto