On optimized builds for all desktop platforms, `nsBlockFrame` is 192 bytes, and
`mFloats` is 16 bytes. With this patch, we can reduce the size of `nsBlockFrame`
to 176 bytes, saving 8.33% memory on every `nsBlockFrame` and its derived
subclasses allocated.
On a real webpage like https://en.wikipedia.org/wiki/Firefox, `about:memory`
shows that we allocate 0.36 MB of `nsBlockFrame` consistently. After this patch,
we allocate 0.33 MB, saving approximately 30 KB (0.36 MB * 8.33%) of the memory.
Differential Revision: https://phabricator.services.mozilla.com/D214046
cd80eb1aef