Previously, `AnonymousBuffer::create_with_size(0)` returned an error
because POSIX `mmap` rejects a zero length with `EINVAL`, and Windows
`CreateFileMapping` rejects a zero maximum size for an anonymous
mapping. This caused a crash when using `--headless=text` with zero
size pages like `about:blank`.
size >> 31 >> 1 is used instead of size >> 32 to support 32-bit Windows
(size_t is 32 bit there, and you cannot shift 32-bit value by 32 bits
on x86)
This is equivalent to sizeof(size) == 4 ? 0 : size >> 32
