size_t is the correct data type for this purpose. Our APIs (in particular
ExternalSourceStream::GetMoreData) are already using it, and there were some
static_casts to convert between them.

This CL doesn't intend to fix all of V8, just the minimal sense-making part
around scanner character streams.

BUG=

Review URL: https://codereview.chromium.org/864273005

Cr-Commit-Position: refs/heads/master@{#26449}