Add an experiment group via Settings to tokenize synchronously

This patch adds an experiment group that tokenizes synchronously
on the main thread in response to a recieve data IPC.

If this regresses performance, we can augment the CL slightly
to only synchronously tokenize when the main parser has no
work.

BUG=627161
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_site_isolation

Review-Url: https://codereview.chromium.org/2139703003
Cr-Commit-Position: refs/heads/master@{#405035}
3 files changed