Restore in-memory parser cache for V8 compile.

Also do a minor refactoring of the decision logic, which hopefully
makes it a bit easier to understand.

This is a fix for the performance regression introduced by
crrev.com/432273004, which inadvertently disabled the parser cache
when compiling without any compile options.

R=marja@chromium.org
BUG=404622
BUG=399580

Review URL: https://codereview.chromium.org/528013002

git-svn-id: svn://svn.chromium.org/blink/trunk@181472 bbb929c8-8fbe-4397-9dbb-9b2b20218538