[XRay] Record the XRay data 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 data to
a file.

[mcgrathr]

lgtm

compiler-rt/lib/xray/xray_utils.cpp

void LogWriter::WriteAll(const char *Begin, const char *End) XRAY_NEVER_INSTRUMENT {
 
   // Write the data into VMO.
   zx_status_t Status = _zx_vmo_write(Vmo, Begin, Offset, TotalBytes);
   if (Status != ZX_OK) {
     Report("Failed to write: %s\n", _zx_status_get_string(Status));
     return;
   }
   Offset += TotalBytes;
+
+  // Record the data size as a property of the VMO.
+  _zx_object_set_property(Vmo, ZX_PROP_VMO_CONTENT_SIZE,
+                          &Offset, sizeof(Offset));
 }
 
 void LogWriter::Flush() XRAY_NEVER_INSTRUMENT {
   // Nothing to do here since WriteAll writes directly into the VMO.
 }
 
 LogWriter *LogWriter::Open() XRAY_NEVER_INSTRUMENT {
   // Create VMO to hold the profile data.