Add relevant traces to measure copyFromCompositingSurface Turn-around time.

content/common/gpu/client/gl_helper.cc

Index: content/common/gpu/client/gl_helper.cc
diff --git a/content/common/gpu/client/gl_helper.cc b/content/common/gpu/client/gl_helper.cc
index c3f9f37e50280baf09553913a36ed30e4b6aa850..eac9e12ffec65c3e6a6855282b1e669baa85a24b 100644
--- a/content/common/gpu/client/gl_helper.cc
+++ b/content/common/gpu/client/gl_helper.cc
@@ -413,6 +413,7 @@ void GLHelper::CopyTextureToImpl::ReadbackAsync(
     GLenum type,
     size_t bytes_per_pixel,
     const base::Callback<void(bool)>& callback) {
+  TRACE_EVENT0("mirror", "GLHelper::CopyTextureToImpl::ReadbackAsync");
   Request* request =
       new Request(dst_size, bytes_per_row, row_stride_bytes, out, callback);
   request_queue_.push(request);