[memory] Shrink ShapeResult by 4-bytes.

The num_glyphs_ field isn't particularly hot (only used in a couple of
places), just look at the runs directly.

This also means we don't need to maintain the field.

There should be no behaviour change.

Change-Id: I3e9a26a76986b8ca5d46d8a07836955dc8373ab8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6476511
Reviewed-by: Koji Ishii <kojii@chromium.org>
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1457004}