Embedders would use these for features which must be able to be turned
off at runtime, despite being compiled into V8. They can be turned on
and off by the embedder using the --experimental_extras flag, e.g. via
v8::SetFlagsFromString.

R=yangguo@chromium.org, mlippautz@chromium.org, hpayer@chromium.org
BUG=chromium:507137
LOG=Y

Review URL: https://codereview.chromium.org/1284413002

Cr-Commit-Position: refs/heads/master@{#30260}