Downside: this adds all kinds of weird includes in the .cc files.
(See design doc linked in the bug.)
BUG=v8:5402
Review-Url: https://codereview.chromium.org/2622503002
Cr-Commit-Position: refs/heads/master@{#42140}
Rebuilding (after touching certain files) is crazy slow because
includes are out of control. Many of these files we need to rebuild are
cctests which pull in more includes than they need.
BUG=v8:5294
Review-Url: https://codereview.chromium.org/2304553002
Cr-Commit-Position: refs/heads/master@{#39080}