As discussed offline, this header is not really useful without its
inline definitions. So instead of trying to split it (unnaturally),
declare defeat and make it an inline header file.

Also remove two unnecessary uses of it, move the JSON stringifier
class into its .cc file (because it needs IncrementalStringBuilder
inline) and forward declare in prettyprinter.h.

R=petermarshall@chromium.org, mstarzinger@chromium.org
BUG=v8:7965

Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: I7ba032ca718a195e156ec3e59f8e858a25423450
Reviewed-on: https://chromium-review.googlesource.com/1156401Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Commit-Queue: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54818}