[gn] Configure trace_events for v8 stand-alone

BUG=chromium:474921
NOTRY=true

Review-Url: https://codereview.chromium.org/2011493002
Cr-Commit-Position: refs/heads/master@{#36475}

BUILD.gn

@@ -721,8 +721,7 @@ v8_source_set("v8_base") {
   visibility = [ ":*" ]  # Only targets in this file can depend on this.
 
   sources = [
-    # TODO(fmeawad): This needs to be updated to support standalone V8 builds.
-    "../base/trace_event/common/trace_event_common.h",
+    "//base/trace_event/common/trace_event_common.h",
     "include/v8-debug.h",
     "include/v8-experimental.h",
     "include/v8-platform.h",