[profile] Record the profile size as a property of the VMO

[phosek]

Summary

While the VMO size is always page aligned, we can record the content
size as a property and then use this metadata when writing the profile
to a file.

[mcgrathr]

A comment wouldn't hurt.

compiler-rt/lib/profile/InstrProfilingPlatformFuchsia.c

if (IOVecs[I].Data) {
       if (Status != ZX_OK)
         return -1;
     } else if (IOVecs[I].UseZeroPadding) {
       /* Resizing the VMO should zero fill. */
     }
     __llvm_profile_offset += Length;
   }
 
+  /* Record the profile size as a property of the VMO. */
+  _zx_object_set_property(__llvm_profile_vmo, ZX_PROP_VMO_CONTENT_SIZE,
+                          &__llvm_profile_offset,
+                          sizeof(__llvm_profile_offset));
+
   return 0;
 }
 
 static void initVMOWriter(ProfDataWriter *This) {
   This->Write = lprofVMOWriter;
   This->WriterCtx = NULL;
 }