GCC: move attribute to the start of function definition in VAAPI wrapper.

Fix compilation error:
../../base/compiler_specific.h:97:28: error: attributes are not allowed on a function-definition
   97 | #define WARN_UNUSED_RESULT __attribute__((warn_unused_result))
      |                            ^~~~~~~~~~~~~
../../media/gpu/vaapi/vaapi_wrapper.h:322:36: note: in expansion of macro ‘WARN_UNUSED_RESULT’
  322 |                     const T* data) WARN_UNUSED_RESULT {
      |                                    ^~~~~~~~~~~~~~~~~~

Bug: 819294
Change-Id: I24e2e10a96182f8705fed96164550357acdb55a4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2534931
Commit-Queue: Miguel Casas <mcasas@chromium.org>
Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Reviewed-by: Miguel Casas <mcasas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#831923}

media/gpu/vaapi/vaapi_wrapper.h

@@ -345,8 +345,8 @@ class MEDIA_GPU_EXPORT VaapiWrapper
   // Convenient templatized version of SubmitBuffer() where |size| is deduced to
   // be the size of the type of |*data|.
   template <typename T>
-  bool SubmitBuffer(VABufferType va_buffer_type,
-                    const T* data) WARN_UNUSED_RESULT {
+  bool WARN_UNUSED_RESULT SubmitBuffer(VABufferType va_buffer_type,
+                                       const T* data) {
     return SubmitBuffer(va_buffer_type, sizeof(T), data);
   }
   // Batch-version of SubmitBuffer(), where the lock for accessing libva is