Reland "[tracing] allow dynamic control of tracing"

This is a reland of bf5ea813

Original change's description:
> [tracing] allow dynamic control of tracing
> 
> If the trace_buffer_ was null, we were returning a pointer to a static
> flag back that permanently disabled that particular trace point.
> 
> This implied an assumption that tracing will be statically enabled at
> process startup, and once it is disabled, it will never be enabled
> again. On Node.js side we want to dynamically enable/disable tracing as per
> programmer intent.
> 
> Change-Id: Ic7a7839b8450ab5c356d85e8e0826f42824907f4
> Reviewed-on: https://chromium-review.googlesource.com/1161518
> Reviewed-by: Yang Guo <yangguo@chromium.org>
> Commit-Queue: Ali Ijaz Sheikh <ofrobots@google.com>
> Cr-Commit-Position: refs/heads/master@{#54903}

Change-Id: Ie2a9ebbaf59d3be1918864cc3c192671fb5550c3
Reviewed-on: https://chromium-review.googlesource.com/1188529
Commit-Queue: Ali Ijaz Sheikh <ofrobots@google.com>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56007}

src/libplatform/tracing/tracing-controller.cc

@@ -25,18 +25,17 @@ namespace tracing {
 // convert internally to determine the category name from the char enabled
 // pointer.
 const char* g_category_groups[MAX_CATEGORY_GROUPS] = {
-    "toplevel", "tracing already shutdown",
+    "toplevel",
     "tracing categories exhausted; must increase MAX_CATEGORY_GROUPS",
     "__metadata"};
 
 // The enabled flag is char instead of bool so that the API can be used from C.
 unsigned char g_category_group_enabled[MAX_CATEGORY_GROUPS] = {0};
 // Indexes here have to match the g_category_groups array indexes above.
-const int g_category_already_shutdown = 1;
-const int g_category_categories_exhausted = 2;
+const int g_category_categories_exhausted = 1;
 // Metadata category not used in V8.
-// const int g_category_metadata = 3;
-const int g_num_builtin_categories = 4;
+// const int g_category_metadata = 2;
+const int g_num_builtin_categories = 3;
 
 // Skip default categories.
 v8::base::AtomicWord g_category_index = g_num_builtin_categories;
@@ -121,10 +120,6 @@ void TracingController::UpdateTraceEventDuration(
 
 const uint8_t* TracingController::GetCategoryGroupEnabled(
     const char* category_group) {
-  if (!trace_buffer_) {
-    DCHECK(!g_category_group_enabled[g_category_already_shutdown]);
-    return &g_category_group_enabled[g_category_already_shutdown];
-  }
   return GetCategoryGroupEnabledInternal(category_group);
 }