Mismatch metric name for CSS property "reading-flow"

[dizhang168]

The CSS property "reading-flow" was initially named "reading-order-items".
It was updated by this patch, in file tools/metrics/histograms/enums.xml:
https://chromium-review.googlesource.com/c/chromium/src/+/5631986

However, the metric id 787 is still mapping to the old name "reading-order-items" instead of the correct name "reading-flow".
https://chromestatus.com/metrics/css/timeline/popularity/787