Fix 64 bit truncation in tracing

BuitinCategories::Size() returned size_t but the ::At(..) method took
an int. Make these consistent to avoid a 64 bit truncation.

BUG=879657

Change-Id: I7e27f17b709f7070a5234ac53b96438efad0eaf3
Reviewed-on: https://chromium-review.googlesource.com/c/1347570Reviewed-by: Primiano Tucci <primiano@chromium.org>
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611724}

base/trace_event/builtin_categories.h

@@ -249,7 +249,7 @@ static_assert(!StrEqConstexpr("abc", "ab"), "strings should not be equal");
 class BuiltinCategories {
  public:
   // Returns a built-in category name at |index| in the registry.
-  static constexpr const char* At(int index) {
+  static constexpr const char* At(size_t index) {
     return kBuiltinCategories[index];
   }