Move V8 GC trace events to their own catergory.

This will allow us to enable GC events without enabling IC events.
Telemetry currently cannot handle huge number of IC events in a trace.

BUG=

Review-Url: https://codereview.chromium.org/1924333003
Cr-Commit-Position: refs/heads/master@{#35902}

src/heap/gc-tracer.h

@@ -112,7 +112,7 @@ enum ScavengeSpeedMode { kForAllObjects, kForSurvivedObjects };
 #define TRACE_GC(tracer, scope_id)                             \
   GCTracer::Scope::ScopeId gc_tracer_scope_id(scope_id);       \
   GCTracer::Scope gc_tracer_scope(tracer, gc_tracer_scope_id); \
-  TRACE_EVENT0(TRACE_DISABLED_BY_DEFAULT("v8"),                \
+  TRACE_EVENT0(TRACE_DISABLED_BY_DEFAULT("v8.gc"),             \
                GCTracer::Scope::Name(gc_tracer_scope_id))
 
 // GCTracer collects and prints ONE line after each garbage collector