[tracing] Avoid Gcc compilation fail by declaring AddTraceEvent function in Class derived from Platform Class.

  The CL #39789 (https://codereview.chromium.org/2367603002 ) caused the Gcc compilation fail for v8 debug mode.
  The error message was:
  In file included from .././include/libplatform/v8-tracing.h:13:0,
                   from .././src/libplatform/default-platform.h:14,
                   from ../src/libplatform/default-platform.cc:5:
  .././include/v8-platform.h:169:20: error: ‘virtual uint64_t v8::Platform::AddTraceEvent(char, const uint8_t*, const char*, const char*, uint64_t, uint64_t, int32_t, const char**, const uint8_t*, const uint64_t*, unsigned int)’ was hidden [-Werror=overloaded-virtual]
     virtual uint64_t AddTraceEvent(
                      ^
  In file included from ../src/libplatform/default-platform.cc:5:0:
  .././src/libplatform/default-platform.h:55:12: error:   by ‘virtual uint64_t v8::platform::DefaultPlatform::AddTraceEvent(char, const uint8_t*, const char*, const char*, uint64_t, uint64_t, int32_t, const char**, const uint8_t*, const uint64_t*, std::unique_ptr<v8::ConvertableToTraceFormat>*, unsigned int)’ [-Werror=overloaded-virtual]
     uint64_t AddTraceEvent(
              ^

  This CL fixed this issue by adding "using Platform::AddTraceEvent;" before all declarations of AddTraceEvent functions in Classes derived from Platform Class.

BUG=

Review-Url: https://codereview.chromium.org/2380583002
Cr-Commit-Position: refs/heads/master@{#39810}