Move trace_event.h include from v8_platform.h to .cc file

trace_event.h is redundant in header file. It also causes compilation
issues when Blink precompiled headers are used on Windows (Bug 495697).

BUG=

Review URL: https://codereview.chromium.org/1585693002

Cr-Commit-Position: refs/heads/master@{#371801}

gin/public/v8_platform.h

@@ -8,7 +8,6 @@
 #include "base/compiler_specific.h"
 #include "base/lazy_instance.h"
 #include "base/macros.h"
-#include "base/trace_event/trace_event.h"
 #include "gin/gin_export.h"
 #include "v8/include/v8-platform.h"
 

gin/v8_platform.cc

@@ -7,6 +7,7 @@
 #include "base/bind.h"
 #include "base/location.h"
 #include "base/threading/worker_pool.h"
+#include "base/trace_event/trace_event.h"
 #include "gin/per_isolate_data.h"
 
 namespace gin {